No BMP/Farbfeld encoder

[hillin]

I was curious of the performance comparison amongst codecs of different image formats that zune-image supports. Apparently BMP and Farbfeld has no encoding support right now. (Not that I need it, just firing an issue for completeness.)

There seems to be an encoder for Farbfeld, but not supported in write_to_vec.

[etemesi254]

more of time constraints, BMP is easy to write an encoder for, but it's not that interesting for me to pursue,but ideally it should be there, the farbfeld one is weird

Also to note, the dev branch overhauled how io is being done, making writes be in anything that implements a certain trait, if you are testing such it would be nice to test on the new traits

particularly I'd love to see tests that see how long it takes to encode to a & mut Vec and also & mut Bufreader <File>