Add a selector for filtering based on custom gitignore patterns

Add a `SelectGitignore` selector that can be constructed from either an iterable of gitignore patterns or a dict mapping relative subdirs to sequences of gitignore patterns.  Unlike #7, this does not honor gitignore files on the file system; instead, filtering is done using gitignore rules specified in the code.

- Also take an optional `case_sensitive: bool = True` argument?