addition of "link" button type and icon support

See: #166

This adds the ability to have a button that's rendered as an html link but styled to look like a regular button.  This allows for things like a cancel button which is just a link to another page (thereby not requiring any form data to be sent, validated, etc).

Also, while I was looking at it, I added functionality to add a Bootstrap icon to the button.  (It seems like it was half implemented)