Find expression bounds in current editor at caret position.
On each character a <code>fn</code> function will be called and must
return <code>true</code> if current character meets requirements,
<code>false</code> otherwise
@param {IEmmetEditor} editor
@param {Function} fn Function to test each character of expression
@return {Range}
function(editor, fn) {
			var content = String(editor.getContent());
			var il = content.length;
			var exprStart = editor.getCaretPos() - 1;
			var exprEnd = exprStart + 1;
				
			// start by searching left
			while (exprStart >= 0 && fn(content.charAt(exprStart), exprStart, content)) exprStart--;
			
			// then search right
			while (exprEnd < il && fn(content.charAt(exprEnd), exprEnd, content)) exprEnd++;
			
			if (exprEnd > exprStart) {
				return require('range').create([++exprStart, exprEnd]);
			}
		}