Add readouterr_bytes for retrieving captured binary output

[asottile]

Followup to #2266 and #1407 to enable assertions on arbitrary binary output

[coveralls]

Coverage decreased (-0.03%) to 92.744% when pulling 97bf365 on asottile:readouterr_bytes into 0f3d7ac on pytest-dev:features.

[asottile]

Oh hmm, I forgot to test in python2. I'll have to think about this some more...

[RonnyPfannschmidt]

@asottile this is to the point wrt my comments to the last pr, instead of having capture configured for binary, we now get different sets of targeted methods with strange errors,

even if we add those please mark them as provisional, i'm reasonably certain a better capture model makes this mess unnecessary

[asottile]

I'll reopen this once I've reworked python2 support (when I have time!)