allow local config to specify bugs directory location

Allow .git-config in the local directory to override the "bugs" directory, i.e., allow people to rename "bugs" to something else without having to specify -i for every invocation.

Here's a git-specific use-case for this: you want to keep issues in a separate branch so as not to clutter your precious development history. It's annoying to switch back and forth between branches all the time, so you make another repo close of just that branch (or whatever the git lingo is) and put it in a directory somewhere.

If you could then tell ditz that the bugs directory is over there, you could run ditz directly from your coding directory, keep your development history pure, and not worry about switching branches all the time.