Integrate option/flag to use Lipo

[niftylettuce]

Lipo is an alternative/drop-in replacement for Sharp that does not require the user to install dependencies such as libvips. It uses Sharp under the hood as well.

I think it'd be helpful to add a { useSharp: true } option or something for users that want to optionally use Sharp, and then make this package use Lipo out of the box by default? If you did this, obviously would need to do a major semver version bump so as to not affect past users.

See https://github.com/lipojs/lipo. I'm very curious to your thoughts and feedback here. I made the rate limiting generous I think, but I'm open to making it more generous if need be. Really not sure what usage will be like since I just launched it.