Allow specifying an external maskfile.md to use

[jacobdeichert]

Right now mask looks for a maskfile or maskfile.md in the current directory.

We should add a --maskfile <path> option so that mask can refer to another maskfile.md at the given path.

Perhaps it should also look for maskfiles in parent directories until it finds one?

Example use case:
Sometimes i'm working in a subdir of a monorepo project. Being able to put a maskfile at root of the monorepo would be nice if I could call mask from any location in that tree.

[jacobdeichert]

Fixed by #19 🎉

[jacobdeichert]

Perhaps it should also look for maskfiles in parent directories until it finds one?

This still might be useful as a default behavior. I'll need to think about this further.

There's a good alternative for user land right now: make a bash alias to mask that searches up the directory tree for a maskfile.md and then execute mask --maskfile $(pwd)/maskfile.md on that.