Add a command to change the surrounding environment

[r-stein]

This resolves https://github.com/msiniscalchi/atom-latextools/issues/102 similar to the ST PR https://github.com/SublimeText/LaTeXTools/pull/585.

This adds the command C-l, C-S-n to change the surrounding environment. It supports:

• nested environments
• multiple cursors
• exclude definitions in comments and verbatim (easily expandable scope selector/descriptor list)

demo:

[cortner]

I am keen to have this, any chance to merge it?