Add possibility to show/hide spelling and grammatical errors

[troosan]

This pull request allows you set hide spelling and/or grammatical errors.
Simply use it as this

\PhpOffice\PhpWord\Settings::setSpellingErrorsHidden(true);
\PhpOffice\PhpWord\Settings::setGrammaticalErrorsHidden(true);

This corresponds to the following option in Word:

This does not fix issue #542 as I did not update the reader to read the settings.xml file.
Anyone feel free to do so :-)