Create a version of a button that contains an icon

[Dacello]

I'm submitting a ...

• feature request

What is the current behavior?
We currently support different button types, but none of them look right with an icon in them

What is the expected behavior?
We support a nicely styled button with an icon inside it

• Add to the example page
• Add to the props table
• Add any new vars to the vars table

See what @brittanygay has implemented in TPN

More details TBD by @brittanygay

[blazebarsamian]

Examples to make:

• one with an icon on the left of the text
• one with an icon on the right of the text (think "next" and "back" buttons).
• maybe one with a icon centered? (pretty sure this would happen by default if there was only an icon in the button with no text but double check :) )