Add option to rewrite sway/window title

Rewrites window title according to config option "rewrite".
"rewrite" is an object where keys are regular expressions and values are
rewrite rules if the expression matches. Rules may contain references to
captures of the expression. Regex and replacement follow ECMA-script
rules. If no regex matches, the title is left unchanged.

example:
"sway/window": {
  "rewrite": {
    "(.*) - Mozilla Firefox": " $1",
    "(.*) - zsh": " $1",
  }
}

include/modules/sway/window.hpp

@@ -22,6 +22,7 @@ class Window : public ALabel, public sigc::trackable {
   std::tuple<std::size_t, int, std::string, std::string> getFocusedNode(const Json::Value& nodes,
                                                                         std::string&       output);
   void                                                   getTree();
+  std::string                                            rewriteTitle(const std::string& title);
 
   const Bar&       bar_;
   std::string      window_;