Add ability to read gitignore for filters

[zombiezen]

I want a function that parses the .gitignore file format and takes the same arguments as a path filter so that I can reuse my project's existing .gitignore file when creating a build script for my project.

[zombiezen]

This would benefit from 256lights/zb#148. The function can be built without it, but it won't be useful to users otherwise.