Parse the original prompt for cwd and env names

[jankatins]

clink.get_cwd() is returning a string which is differently encoded than what
clink.prompt.value expects. This results in garbled path names if the path
condains non-ASCII chars. The (arguable hacky) solution is to parse the old
prompt for the current directory (which breaks if the user sets a PROMPT env var
which is incompatible to the regex used here...).

Also parse out a environment name set by systems like virtualenv or conda: this
could be done more specifically by targeting each such system and using the
usually set environment variable but this would mean that we would have to do
that for each and every such system out there and that is probably not a sane
idea...

Closes: #1054 (garbled prompt)
Closes: #1057 #1056 (environment issues)

Example session:

C:\Users\jschulz
λ activate py35

C:\Users\jschulz
(py35) λ cd c:\temp\ü\(hallo)\

c:\temp\ü\(hallo)
(py35) λ

A "better" solution would be proper unicode support in clink: mridgers/clink#415

[jing2si]

nice work~

[rehack]

@JanSchulz @jing2si
Good job! Buddy. ⊙o⊙

[Stanzilla]

@JanSchulz do you still want to wait for a response to your clink issue or won't that change what is in this PR?

[jankatins]

@Stanzilla IMO this is a nice shortterm fix for the problem. If there is a better way (=proper unicode support in clink) , I will add a new PR.

[davpaez]

According to this commit in clink, the encoding problem appears to have been fixed.

If so, should the clink.lua script go back to use the clink.get_cwd() inside the set_prompt_filter() function again, instead of relying on the more convoluted parsing of clink.prompt.value?