Escape invalid XML characters when reading in a Word2007 file

[tjarrett]

This fix resolves issue #720 in which reading in and then immediately saving a Word2007 file fails if there are any invalid XML characters in the document.

I realize it is the user's responsibility to escape content when using the Writer (and actually agree with this stance), but it feels like it is this library's responsibility to do so when reading in a document.

To test this I fed xml-characters-in.docx into PHPWord using this code:

$test = \PhpOffice\PhpWord\IOFactory::load('xml-characters-in.docx');
$test->save('xml-characters-out.docx');

And the result is the attached file xml-characters-out.docx.

[troosan]

merged manually