Add fill plugin

[arashm]

This commit adds a new plugin to darkroom which is similar to crop. It allows you to select an area and fill the area black. Useful for when you want to censor a part of the picture.
This is probably not the best production level code, but it works. I tried to avoid making duplicate codes and moved most of the parts related to selecting an area to another plugin so both crop and fill uses the same code. I just leave it here for others who may need similar functionality. If merged, others will have the opportunity to make it better.

[Asuza]

@arashm, thank you for providing this pull request. It gave me a great head start on implementing my own fill functionality. The main difference being that I want the fill to work immediately, rather than having to select and confirm an area.

[phuicy]

Amazing, this is exactly what I was looking for.