Support saving as gif

[twolfson]

As initially posted in #4, it would be nice to have gif support since it is supported in get-pixels. One library that can be used for this is:

https://github.com/twolfson/gif-encoder

[twolfson]

Going to sit on this until #6 and #7 get landed. It can be very tricky working with 3 branches that depend on each other/have conflicts x_x

[mikolalysenko]

Yeah, I appreciate your patience. Thank you so much for all of your PRs! I am sorry that I have been a bit slow getting things merged, was very busy last week with teaching and didn't get much time to hack on stuff.

[twolfson]

No worries, these PRs aren't blocking anything time sensitive.

About this issue: I think with GIFs we are going to need to add support for a 4d array to match the signature of get-pixels.

[numFrames, width, height, 4]

https://github.com/mikolalysenko/get-pixels