Support transparency and reserved colors in the palette

[leeoniya]

It would also be good if rgba(0,0,0,0) pixels are converted to the transparent color

[jnordberg]

(re-worded the issue slightly)

[beager]

This should be fairly straightforward as the GIFEncoder object exposes a setTransparent(number) method. If I can get hip to the coffeescript I might try to mess with this and open a PR.

Also, it seems that setTransparent() refers to a color in the table, not a specific hex color?

[beager]

I have submitted #15 that will allow you to explicitly set a background color when instantiating the encoder. This does not address reserved colors.

As for rgba(0,0,0,0), I'm not sure how all of that would work. All of my experience is with converting a canvas context, which in my experience has some full-alpha color at all times.

[1j01]

:+1: for rgba(0,0,0,0) transparency