Add support for sixel images using libsixel.

Regarding libsixel:

Built at commit 2df64373e58c4f48fbb72032a66473e71fc2a6ce. Here's how I built it:

setenv MACOSX_DEPLOYMENT_TARGET 10.12
make clean && ./configure --without-libcurl --without-jpeg --without-png --prefix=`pwd`/libsixel && make && make install

Then delete everything except the include and the .a library and copy it over.

ThirdParty/Licenses.txt

@@ -566,4 +566,115 @@ The above copyright notice and this permission notice shall be included in all c
 
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
+---
+libsixel/LICENSE
+
+Copyright (c) 2014-2016 Hayaki Saito
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+---
+libsixel/LICENSE.sixel
+
+"sixel" license (original license)
+==================================
+
+src/tosixel.c and src/fromsixel.c are derived from "sixel" original version (2014-3-2)
+
+  Package: http://nanno.dip.jp/softlib/man/rlogin/sixel.tar.gz
+
+  Unofficial repo: https://github.com/saitoha/sixel
+
+This work is written by kmiya@ culti. He distributes it under very permissive license.
+
+The original license text(in Japanese) is:
+
+```
+このプログラム及びソースコードの使用について個人・商用を問わず
+
+ご自由に使用していただいで結構です。
+
+また、配布・転載・紹介もご連絡の必要もありません。
+
+ソースの改変による配布も自由ですが、どのバージョンの改変かを
+
+明記されることを希望します。
+
+バージョン情報が無い場合は、配布物の年月日を明記されることを
+
+希望します。
+
+                  2014/10/05  kmiya
+```
+
+The unofficial translation:
+
+```
+Anyone is free to use this program for any purpose,
+commercial or non-commercial, without any restriction.
+
+Anyone is free to distribute, copy, publish, or 
+advertise this software, without any contact.
+
+Anyone is free to distribute with modification of the
+source code, but I "hope" that its based version or
+date will be written clearly.
+
+                                    2014/10/05 kmiya
+```
+
+kmiya also said this is compatible with MIT/BSD/GPL.
+
+---
+libsixel/LICENSE.xterm
+
+The helper function hls2rgb in src/fromsixel.c is imported from
+graphics.c in Xterm pl#310 originally written by Ross Combs.
+
+http://invisible-island.net/xterm/
+
+
+Copyright 2013,2014 by Ross Combs
+
+                        All Rights Reserved
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be included
+in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+IN NO EVENT SHALL THE ABOVE LISTED COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+Except as contained in this notice, the name(s) of the above copyright
+holders shall not be used in advertising or otherwise to promote the
+sale, use or other dealings in this Software without prior written
+authorization.
+