Currently we have a "split" function which splits a str/bytestr into
chunks of their underlying data. This works great for the most tivial jobs.
But there is no possibility to pass an offset parameter into the split
function which indicates the next "user-defined" starting index.

Actually the next starting position will be build upon the last starting
position (of found sep.) + separator length + 1.

It should be possible to manipulate the next starting index by changing this
behavior into:

last starting position (of found sep.) + separator length + OFFSET.

NOTE: The slicing start index (for substring) stay untouched.

This will help us to solve splitting sequences with one or more consecutive
separators. The following demonstrates the actually behavior.

>>> s = 'abc;;def;hij'
>>> s.split(';')
['abc', '', 'def', 'hij']

This works fine for both str/bytes values.
The following demonstrates an "offset variant" of split function.

>>> s = 'abc;;def;hij'
>>> s.split(';', offset=1)
['abc', ';def', 'hij']

The behavior of maxcount/None sep. parameter should be generate the same
output as before.

A change will be affect (as far as I can see):
- split.h
    - split_char/rsplit_char
    - split/rsplit