Do not escape characters that do not need escaping in CSS string

[8ctopus]

Add draft to attempt to resolve #1443

There are no specific tests yet, and there are improvements that can be made, I just want to get your approval before I polish things up.

[coveralls]

coverage: 68.727% (-0.5%) from 69.191%
when pulling f724347 on 8ctopus:css-string-escaping
into 0ae1fde on MyIntervals:main.

[JakeQZ]

I think it could be significantly slower looping through the string rather than using the built-in function (particularly as some strings may be base64-encoded images). Also this is reimplementing some of what addslashes() does.

Wouldn't it be simpler (and more efficient) to use addslashes() as before, then do a str_replace() to replace \' with ' or \" with " depending on the string quoting type?

[JakeQZ]

Wouldn't it be simpler (and more efficient) to use addslashes() as before, then do a str_replace() to replace \' with ' or \" with " depending on the string quoting type?

PS. I agree with adding a separate method to do this.