scanning query strings with offsets

Added a ``offset`` kw-paramter to ``dawg.prefixes()`` (and ``.iterprefixes()``) to make it possible to iteratively scan a string for prefixes without having to take slices of each prefix of the query. I.e., before::

    dwag = DWAG([...])

    def scan(query:str):
        for i in range(len(query)):
            yield dawg.prefixes(query[i:])

New behavior::

    def scan(query:str):
        for i in range(len(query)):
            yield dawg.prefixes(query, i)