Feat: Support default colours

[NickDJM]

Summary

There are times when you do not want a bunch of different colour shades and only want a single colour and you want that single colour to have a class like bg-primary instead of bg-primary-500.

Would be nice if there was a special _default key that would generate that kind of colour instead of always having the sub-key in the name.

Solution

Support a _default colour key.

Additional context

We have a client who has solid colours defines and no shades. It might be something their particular about.