Parse literal tabs in docx

Turns out DocX has literal tabs as well.

The test for this revealed a problem with the way the reader interprets Spaces at the beginning of paragraphs (it previously left them in). This patchset fixes that, and fixes a previous test which didn't account for it. There are also tests for tabs in the middle of sentences (which turn into spaces, as with markdown input) and at the beginning of paragraphs (which are ignored).