mergefield regex doesn't match some merge fields produced by word

When I used Word 2010 to create a template, the name of the merge field was enclosed by two spaces (`__`), rather than one space (`_`), or space-quote (`_"`), which is what the regex on [line 38](https://github.com/Bouke/docx-mailmerge/blob/master/mailmerge.py#L38) currently looks for.

     MERGEFIELD__fieldname__\* MERGEFORMAT

(I'm using underscores instead of spaces for readability purposes)