Add automated resource loading of icon-images

[julianrex]

See mapbox/mapbox-plugins-android#1000 (comment) and @fabian-guerra's subsequent comment.

Because we use a delegate method to handle this we'd need to either:

• add an object that can intercept and forward delegate methods
• or add hooks to the SDK (either at the core or iOS level) that an extension could use to provide this behavior.

I favor the latter - and this could be considered as a part of a larger extension integration effort, though it would be straight-forward to add a custom delegate object (and even easier just to add something to the SDK).

Another interesting option in the mean-time might be to use class_addMethod to add default functionality to an existing class/delegate - and this could be done via an extension. Though this might be a bit too "magic".

[julianrex]

Closing as stale.