Better validation of style values

[dpvc]

This PR improves the handling of CSS styles, in particular the use of quotation marks and backslashes in CSS style values, and sanitizes CSS values when output as a string, avoiding a potential for CSS insertion. It also replaces newlines by spaces to avoid some unusual parsing by browsers that newlines can cause.

The parsing of a CSS string now handles quoted material properly, and in the case that a value includes an unmatched quotation mark, one is appended to make sure they are matched. When outputting a CSS value, the value is trimmed at the first unquoted semi-colon to avoid the possibility of having the value insert additional unexpected CSS rules.

I will make some tests for this in a separate PR.

[zorkow]

lgtm.
I did some simple manual tests in node to check the patterns. They appear to work as expected. But some detailed tests will be good.