Refactor + better icon support

[martijnimhoff]

Hey folks from PDMFC,

Thanks for making this package. I love it!

This PR is a bit big, but it's backwards compatible and I believe it makes it better: with this update you can now stuff like this:

I've done the following:

• Refactored most Vue component code into a mixin.
• Made the loading state consistent for the index and the detail field.
• Cleaned up not used code.
• Added support for 3rd party icon libraries, such as Fontawesome (based on https://github.com/mirovit/nova-notifications#icons)
• Build code in production mode instead of development mode.
• Made the code styling more consistent.

I'm looking forward to your feedback!