Add calc support for margin and padding

[BramMeerten]

Add support for calc for margin and padding. Other properties such as width, top, ... already support calc.

More properties should support calc, but I haven't included them all in this PR:

• background-position: calc(100% - 0%) calc(100% - 10%). Not included because parse function splits on whitespace, but whitespace can also occur inside calc parentheses.
• border-bottom-width (and other "length" types): uses parseLength (from parsers.js), didn't want to change because too many usages. It could be done though, for example parseMeasurement already allows calc.

This PR should fix https://github.com/jsdom/cssstyle/issues/154

[BramMeerten]

@domenic could you please have a look at this small PR?