Improve tokenization script

* Don't tokenize urls into parts
* Make sure slashes are split into separate tokens
* Remove `.MDxx` directives
Check only one level of parens in URLs

The '*' quantifier in the nested paren part of the URL regex in
`split_sentences.py` caused the script to hang when in encountered
unmatched parens. This commit alters the regex to use the '?' quantifier
instead. The regex also now only accepts one level of parens (it would
be possible to still allow two nested levels of parens, but it would be
more complicated and is not worth the effort).