Support running in a subdirectory of the working tree

[mattmccutchen]

Braid was only designed to work when the current directory is the root of the working tree. Running Braid in a subdirectory currently doesn't work (Braid looks for .braids.json in the subdirectory), and #62 will change Braid to raise an error in this case.

This issue is to support running in a subdirectory properly by looking for .braids.json in the root of the working tree and interpreting mirror paths relative to the root. As discussed in #44, command-line arguments should typically be interpreted relative to the current directory so that the shell's default path completion behavior works.

[YOUR1]

Is this ever gonna be picked up?