add `enabled` config option and deprecate `disabled?` option

[GregMefford]

I think it's kind of confusing to have negative configuration options like disabled?: true rather than enabled: false. I also think it looks weird to have configuration options with question marks at the end, but I might be the one who's wrong on that one. It bothers me less when it's a function name.

I think we could make this change without breaking anything, so it doesn't need to wait for the 3.0.0 milestone.

[zachdaniel]

This should be a relatively easy change to add, we'll just have to respect both flags for a period of time.