Feature: Directory level contributor scanning

[chartgerink]

Hi 👋 As mentioned in #33, I love this package. Been using all-contributors for years and this is just a fantastic package for recognizing contributions! 😊

In my work, I was trying to create one big contributor list across an entire organization, whose repo's I'd cloned locally already. To that end, I needed to scan all the repositories living in a specific folder. I fiddled around with the main function and was able to do this locally, with relatively little effort (it ain't pretty though!).

I wondered whether a more refined implementation of that feature would be of interest to the package? I was imagining something along the lines of add_contributors_dir() but have not thought about it too deeply --- I wanted to touch base with you and see whether this would be a worthwhile contribution first 😊

[mpadge]

Sounds great to me! An even easier option would be just to modify the add_contributors() function. Best idea might be to just update the repo parameter to accept multiple values. Most current code in that function could then be ported off to a sub-function, add_contributors_one_repo or something, and then the main function could loop over repos and collate everything. The destination files can be explicitly specified anyway, so that would probably all work.

It'd be great if you wanted to submit a PR with something along those lines 👍 thanks again 🚀