Allow overriding the Vuetify config

We want to allow people to override the Vuetify configuration for two purposes:

1. Allows us to give users a better sense of control/flexibility with theming.
2. Makes this package more compelling to potentially push upstream in the future.

I want to let someone define their own vuetify_config.json and if they do then merge it into the global one (with their settings taking precedence). I also want to add a couple of shortcuts to allow them to just define primary, secondary, and accent colors.