Add formatUrl transform to magic button

[fraddski]

Adds a new transform formatUrl to the magic button, as well as a new optional transformArg property which is passed to transforms. This allows the button to take a field and transform it to a url of a given format.

This is required for the episode recap component for Vulture, which needs a magic button to automatically create an image url based on the show name.

[nelsonpecora]

Looks good! Could you add this new argument to the magic-button.md docs before this is merged?

[coveralls]

Coverage increased (+1.2%) to 76.872% when pulling 1d0c1d5 on magic-button-format into 8d8eb6b on master.