see github issue #8

see #7

This also includes a minor refactoring of the code to be less duplicative and rely on multiple values instead of list return values.

The idea behind the ``` marker is to toggle the preformatted bit on and off, so it's not strictly speaking a "verbatim block", even if it's easier to manage it that way. However, don't throw generic errors like that, and handle EOF without a closing ``` marker. While here, if label is the empty string, use nil.

This adds: - :type options. - :documentation options. - :accessor options. - Class predicates. Wherever applicable.

`=>` without other fields is not a valid link line, so let's handle it by parsing it as a paragraph with the "=>" text in it.