Give splitter classes an option to split on maximal consecutive occurrences of the separator

- Possible names for the option:
    - `greedy`
    - `repeat`
    - `multi`
    - `max_span`
    - `munch_all`
    - `split_on_all`
    - `split_max`
    - `maxsep`
    - `span`
    - `maximize`
    - `merge`