cli: add support for path globbing to ignore_paths

[willcl-ark]

I'd like to use globbing as part of ignore paths, I think this does the job, so long as you don't ming adding the glob dependency.

Breaking out into functions felt easier and avoided some duplication I noticed writing it.

[becheran]

I am not against it. I think globbing actually is a good idea for the ignore path. Though, the PR is not complete yet right? I see that you added a test in file_traversal.rs, but the existing logic of ignore_path of the mlc application was not changed?

[willcl-ark]

Thanks. It was pointed out to me since, and I think I agree, that actually the more useful (and probably more typical!) use-case here would be ignoring all files ignored by git. I started implementing this in another branch:

https://github.com/willcl-ark/mlc/tree/git-aware

It's working well enough for me, but I haven't yet implemented having it search for only markup_types (if specified).

Would be curious to know what you thought of this approach vs globbing? IMO it's more robust to updates (in git repos, where I suppose this is mostly used) than glob arguments.

[willcl-ark]

Closing in favour of #94