expand variables in path

Closes https://github.com/srusskih/SublimeJEDI/issues/159

Modified function operates on dict `subl_vars` with keys as in [build-system-variables](http://docs.sublimetext.info/en/latest/reference/build_systems.html#build-system-variables) and according values.

It also expands OS env vars, which all are tested to work correctly on Linux and Windows.

Maybe this problem can be approached also differently like in SublimeRepl build hack - calling daemon though custom build command (if possible, I haven't looked at it) that would accept ST build variables that would further allow using placeholders and substitutions.
We want to be sure that key is not None, so both should stay