Accept CR as an alias for LF to support more platforms

[clue]

The enter key will usually end the line with a \n (LF) character on most Unix platforms. Common terminals also accept the ^M (CR) key in place of the ^J (LF) key.

By now allowing CR as an alias for LF in this library, we can significantly improve compatibility with this common usage pattern and improve platform support. In particular, some platforms use different TTY settings (icrnl, igncr and family) and depending on these settings emit different EOL characters. This fixes issues where enter was not properly detected when using ext-readline on Mac OS X, Android and others.

Note that Windows technically uses \r\n (CRLF), but this platform is not supported at the moment anyway (#18). I've actually implemented logic to ensure CRLF is only handled as a single line, but reverted this for the PR because this does not seem to be implemented in common terminals either (echo -en "\r\n" | bash -i shows two independent lines and both ^J and ^M show a \n in hd).

Resolves / closes #66
Resolves / closes #77
Supersedes #73

[clue]

For the reference: I've just reverted #73 via #80 now this PR addresses the underlying issues. This means that ext-readline will now be used again on all platforms that support it, including Mac OS X.