Add BGRA and BGR support.

[quadrupleslap]

Also allows TGA and PPM encoders to be used with them.

Resolves #451 (I think.)

[nwin]

I think it would be a better option to convert it to RGB for now. I'm hesitant adding new values to that enumeration since it changes a lot of public API.

[quadrupleslap]

Fair enough, but can you at least check over the PPM encoder? It's short, but very strangely written, e.g. let _ = try!(...); is everywhere, and I'm not sure why it's calling write_all three times with an array of one element, instead of once with an array of three elements.

[bvssvni]

The PPM encoder was changed in #730, so I think this should be closed. Reopen if you want to make another attempt.

[quadrupleslap]

That's fine, I don't use this library any more, but in any case:

• Not many formats natively support BGRA, so a conversion has to happen somewhere.
• Conversions probably belong in another library, not this one.

So not adding BGRA was probably the right decision.