A simple string tokenizer which takes a string and splits it on whitespace.
It also optionally takes a string of characters to use as delimiters, and
returns them with the token set as well. This allows for splitting the
string in many different ways.

This is a very basic tokenizer, so more complex needs should be either
addressed with a custom written tokenizer or post-processing of the output
generated by this module. Basically, this will not fill everyones needs,
but it spans a gap between simple 'split / /, $string' and the other
options that involve much larger and complex modules.

Also note that this is not a lexical analyser. Many people confuse
tokenization with lexical analysis. A tokenizer mearly splits its input
into specific chunks, a lexical analyzer classifies those chunks. Sometimes
these two steps are combined, but not here.