Re-add support for `g:lua_path`

Commit 6d68f3d removed the functionality to define/override $LUA_PATH,
but the documentation still mentions it.

This also adds support for caching in xolox#lua#getsearchpath, which
appears to be useful anyway.

I could also imagine a way to support extending the path (prepending and/or appending to it).

My use case is the awesome window manager, which adjusts its path internally, and changes are not reflected in `$LUA_PATH` or the lua interpreter.