Be more flexible towards accepting "LineRule"s

Modified `lineType` predicate so it recognizes any number of chars belonging to {*, _, -} greater or equal than three, or spaces, as a `LineRule`.

This makes it recognize things such as `*********************` as an horizontal line,  as other parsers such as Showdown or Github's do, eg:

*********************
---------------- --------
_ _ _ _ _ _ _ _ _ _ _