New Feature: centering views together

[markrickert]

This feature allows you to select multiple views (that must be contained within the same superview or the feature will not work) and center them in their superview either vertically or horizontally in that superview.

Example usage:

rmq(:view_1, :view_2).center(:vertical)

This will center both views in the superview keeping their current vertical separation.

It's basically group centering. :)

With comprehensive tests.

[markrickert]

[andrewhavens]

Looks okay to me 👍