Feature Request: Handle Whitespace in Equation Marks

I write TeX math in XML documents, e.g. like so:

<inlineequation>
  <mathphrase role="tex">
    e = mc^2
  </mathphrase>
</inlineequation>

The Equation Marks include all these (and similar) start and end tags. They may contain arbitrary runs of white space, possibly including line breaks. However, the current code supports only literal matches.

I suggest that a space character in an Equation Mark match any number of whitespace characters, including zero.

More generally, one might allow regexps for Equation Marks. However, it seems to me that treating white space specially is easier both for the implementer and for the users, while addressing the vast majority of use cases.

If you think this is a good idea I may one day implement it myself and send a merge request.

Great package btw!