Add support for ancestor folder location for Rebracer.xml

[kzu]

It's not uncommon for a repository to contain multiple solutions at
multiple directories and depths within the repo. Instead of having
to maintain a plethora of Rebracer.xml files alongside each .sln,
allow the extension to walk the directories up to find a settings
file.

This may be a performance issues with super deep paths, but
everyone knows that's a bit issue with MAX_PATH in Windows
anyways so typically there aren't that many directories to traverse.