Revisit constants and configurability

[mtscout6]

Issue #306 highlights the ability to modify the constants used for prop validation and mapping. The name constants is deceiving since they are not truly constant values. We should give some thought to this practice give it a proper name. When we do this we should add some documentation for those wishing to work with custom classes. Issue #404 is a fine example of such a case.

[trevorr]

Perhaps the right fix is to add isKeyOf to CustomPropTypes and use that within the propTypes definition of BootstrapMixin, so that additions to STYLES are used? I think it would also be better to add a function to BootstrapMixin to define new styles, rather than modifying constants. That would provide more control (e.g. add but not remove), clarity (STYLES being a map is strange from an external perspective), and debuggability (a place to set a breakpoint when styles are added), and it's slightly less of an exposed implementation detail. It might be reasonable to then make constants private again. Thoughts?

[AlexKVal]

Superceded by #646