Add constructor option for transparent color

This pull request adds the ability to specify a color to be treated as the "transparent" part of the GIF. You specify this upon instantiation like so:

```
var encoder = new GIF({
  workers: 2,
  quality: 10,
  transparent: 0x00FF00
});
```

This will set the transparent color to green, and when the GIF renders, any frame pixels that are `#00ff00` will be transparent instead.

Potential issues:
* I have not tested this against renders that coerce frames down to a certain color palette, so I don't know if this would cause any issues with that. But there is no change to `GIFEncoder.js`, so I can't imaging anything would not work as intended.