Fix #22, use customView of UIBarButtonItem instead

[CaliCastle]

Checklist

• I've tested my changes.
• I've read the Contribution Guidelines.
• I've updated the documentation if necessary.

[raxityo]

Hi @CaliCastle, most probably you're already aware of this because the PR is still open, but unfortunately this approach won't work for system buttons because there's no API to get the frame of UIBarButtonBarItem without a customView.

Perhaps the alternative to AppStore rejection is to always use UIBarButtonBarItem with customView?

[CaliCastle]

@raxityo yeah I was aware of that and it's kinda the only way to accommodate usage with UIBarButtonItem, it's only possible to get the frame and position attributes using the private API without customView so I guess it makes sense to force a customView then