Feature request #2181: Support per-directory settings overrides

Not sure whether these would be in the form of .command-t.some-extension files in directories (seeing as we're already potentially doing an upwards traversal for looking for an SCM root) or a map of settings you could add in your ~/.vimrc.

The use case here is primarily driven by working in submodules. We'll traverse up into the superproject, and you'll end up searching a much larger search space. In the case of my dotfiles repo it's even worse, because if I am working in ~/.vim/bundle/command-t, ~/.vim is actually a symlink to ~/code/wincent/.vim; ~/code/wincent/.vim/bundle/command-t is a submodule of ~/code/wincent, and that gets used as the root. So not only am I searching for more than I want (including any open files which will be mirrored as ~/.vim/tmp/swap/some_file.rb.swp), the files I am trying to find are also underneath a dot-directory, so I have to prefix my searches with a period to get the files to show up.

I'll keep my mind to alternative workarounds though, like the ability to stop traversal if inside a submodule. (Actually, in submodules .git will still exist but in recent versions it will be a normal file containing a "gitlink"; eg: "gitdir: ../../../.git/modules/.vim/bundle/command-t", so I could just check for existence rather than directory-ness and I think we'll be good. I'll leave this issue open anyway because there still might be a worthwhile use case for per-directory overrides.)