Add `overview()` as a high-level overview of a fitted GAM

[gavinsimpson]

This should be a generic with methods for the model types we have

Columns:

• term is the name of the term. For smooths this is the smooth label: but what to do about parametric terms - keep them as individual terms for factors or collapse back to the variable?
• type is the type of term. For a smooth it would be the type of smooth. For a parametric term it could be "parametric", or "linear" or "categorical" ("factor") if we want to distinguish these
• edf is the term's effective degrees of freedom

More?

[gavinsimpson]

This is added now with methods for models fitted by gam(), bam(), and gamm() currently provided.