Normalize to RGBA when decoding indexed images

[mourner]

@targos Thanks for such a nice library! Just sharing something I found unexpected (after trying to switch to fast-png from pngjs):

• If you want to manipulate pixel data after decoding an indexed image, you have to normalize it into RGBA (from palette + indices) manually. This is 1) not documented, 2) inconvenient. Maybe worth adding a normalize option to decode?
• The format of the palette property is [[r, g, b], [r, g, b]], which is inefficient — allocating more memory than necessary. Since we know the number of colors before hand, ideally this would be returned in flat typed form — Uint8ClampedArray([r, g, b, r, g, b, ...]).

[targos]

• A normalize options sounds good. Do you think it should be a general option (that would normalize any image to 32-bit RGBA) or just something that handles palette conversion? Would you like to work on it?
• It is true that using an array of arrays takes more memory. But I'm not sure it's worth changing it, because the palette may contain at most 256 entries.