Apply API improvements, unexport unneeded symbol.

- API improvements as described in #6.
- Unexport `keywords` map as it's not expected to be used by clients and is not documented. This makes the packages API slightly smaller and easier to use.
- Closes #6.

Unfortunately this requires a trivial change (improvement) for `Printer` and `Annotator` interfaces. Which required me to update my usages in a few packages, but it really took less than 3 minutes (by hand, in lieu of a better `go fix`-like solution :( ).

(Btw, I still don't have push rights here so I had to fork.)