Add escapecss filter (#3546)

* add escapecss filter

this filter would allow creating valid css classes from titles containing special characters

we assign a class to an element using `encodeuricomponent[]` so that the class name is encoded

in a stylesheet we create the classname by `<title>escapecss[]` which applies the uri encoding and escapes characters that need to be escaped

* Update encodings.js

* refactor tagToCssSelector, add escapeCssSelector

* use escapeCssSelector

* escape using CSS.escape if it's available

* Update encodings.js

* revert factoring out escapeCssSelector

core/modules/filters/encodings.js

@@ -98,4 +98,13 @@ exports.escaperegexp = function(source,operator,options) {
 	return results;
 };
 
+exports.escapecss = function(source,operator,options) {
+	var results = [];
+	source(function(tiddler,title) {
+		// escape any character with a special meaning in CSS using CSS.escape()
+		results.push(CSS.escape(title));
+	});
+	return results;
+};
+
 })();