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.