Font Awesome 5 support

[jakezatecky]

Currently this component largely makes use of Font Awesome 4 icons (assuming a developer has not opted to use the icons property). While the same set of icons are generally available in Font Awesome 5 Free, some of their Unicode values have changed and there is a delineation between "regular" and "solid" styles of each icon.

The upgrade path is relatively straightforward, but we likely want to continue to support the old Font Awesome 4 style. Due to backward compatibility considerations, we would want to default to the older FA4 while providing an easy way to using FA5.

This could be through the use of a property like iconsClass, which could be set to 'fa5'.

On the v2.0 release, it might make sense to make Font Awesome 5 the default.