allow custom feedback icons

[jeffuyeno]

So... we don't use bootstrap glyphicons... this seems like an edge case, but it does increase happiness.

• add custom template parameter feedback-template
• add valid-icon
• add invalid-icon
• add tests
• add docs on usage

[jeffuyeno]

So... I was thinking about this.... no one is going to want to configure every single instance of form controls.... I'll need to add a configuration service so we can just set the icon types and the feedback template