Add removeDisplayNone option + tests

This adds an options parameter and some tests for a `removeDisplayNone` option
which specifies if the Emogrifier parser will remove elements with the style
"display: none".

Unfortunatly, that nice little optimization had some consequences.

When writing responsive email templates, "display: none" is used often to hide
certain sections of the template for desktop email clients and show them in
phone clients.

I can see how it'd be nice to clean up elements that will not be displayed in
some cases, so I made it an option.
Why the removeChild check?