Add XRegExp dependency

[Jaifroid]

As a result of #179, and following the example of @mossroy's initial attempt to fix that in commit 25c34b6, we could add the XRegExp library as a dependency. For now, all this would do is to avoid having to maintain the Unicode punctuation regex in util.js., but it could be useful more broadly in the future since it patches RegExp support to provide many modern utilities.

[Jaifroid]

While it's a nice library, there are higher priorities, and I'm very comfortable using native regular expressions.