Issue pugjs#473 Allow multline mixin arguments.

lib/lexer.js

@@ -369,7 +369,7 @@ Lexer.prototype = {
 
   mixin: function(){
     var captures;
-    if (captures = /^mixin +([-\w]+)(?: *\((.*)\))?/.exec(this.input)) {
+    if (captures = /^mixin +([-\w]+)(?: *\(((?:(\(.*?\))|(?:"[\s\S]*?")|(?:'[\s\S]*?')|(?:[\s\S])*?)*?)\))/.exec(this.input)) {
       this.consume(captures[0].length);
       var tok = this.tok('mixin', captures[1]);
       tok.args = captures[2];