Add typedef tag

With this change we introduce a new tag called typedef.
A powerful tag that allows you to define new types on the fly
and augment existing types.

See https://github.com/phpDocumentor/fig-standards/issues/7 for the discussion
> [...] such as methods or properties that could or were not documented in the original.

I think something must have been left out here. Or maybe left in when refactoring...