Add button support

[dylanslewis]

This is a bit difficult from Sketch, since it doesn't have a very good template for button symbols well defined within the format. It might be better suited to exports from Lona or InVision DSM, although a Sketch extension could be used to define this type.

Buttons are also much more difficult to have a one-size-fits-all approach for templates, since there are many different types of button, including:

• Text
• Image
• Borders
• Element placement
• Highlighting behaviour