New custom mode: Arrays

[janhaa]

Hi,

I started working on a new mode to create arrays of features in one go.

I mostly only need the rectangles for now, so if you want to contribute with new features, you can find the repo here: https://github.com/janhaa/mapbox-gl-draw-arraymode

[adrianababakanian]

Thanks for your contribution @janhaa ! If you'd like, you can open a PR linking out to the repo for your custom mode in docs/MODES.md, as noted here, to close out this issue.