Issue 10367 - DDoc should output enum base types.

http://d.puremagic.com/issues/show_bug.cgi?id=10367

A new macro is introduced that's called `DDOC_ENUM_BASETYPE`, it's used to wrap the base type of the enum. This will allow filtering the type and optionally make the text linkable. 

The example in `ddoc10367.d` uses `ddoc10367.ddoc` to highlight the enum base type in a red color.