baseClass option needs to be sanity checked

[mainerror]

The baseClass option needs to be sanity checked in order to prevent setting invalid CSS identifiers. There are three forms of invalid CSS identifiers:

• starting with a digit, .0my-class
• starting with a double hyphen, --my-class
• starting with a hyphen followed by a digit, -0my-class

There are three options I can see how we could handle this:

1. The graceless way - throw an error and inform the user.
2. The graceful way - use the default class naming and inform the user.
3. The silent way - silently use the default class naming and not inform the user.

I'm all for option two informing the user via a console message.

[rlemon]

I'm not sure option 2 makes sense for this application;
If the user defines a baseClass name, one can assume they are using said name in their corresponding css / js selectors.
If we fallback to the default names, I think that'll be as useful as not providing a classname for the span elements all together. Just a thought. Personally I would go with option one. Throw an exception in the console, and abort, or use NO classnames?