Allow customization of the UltiSnips search path on a per-buffer basis.

After talking to SirVer about how to customize the UltiSnips search path
on a project basis, he suggested the technique implemented by this
patch.  It allows you to push something like the following into your
.lvimrc:

    let b:UltiSnipsSnippetDirectories = ["UltiSnips", "Git"]

Where "Git" would contain snippets customized for working on Git in this
example.