Get avatar URL

[JoryHogeveen]

Currently we only support get_avatar() in Pods::field() or {@avatar} magic tags.
We should also support avatar_url or avatar_src as a field input.

[jimtrue]

@JoryHogeveen Shouldn't these just use the _img or _src helpers?

[JoryHogeveen]

@jimtrue thats even better :)