Use JavaScriptCore for evaluation, with Uzbl.run()

Use JavaScriptCore for evaluation, with Uzbl.run()

Both "js" and "script" commands use eval_js(), which creates an instance
of the Uzbl object class (created by js_init()) which right now provides
only a run() method (handled by js_run_command()). After evaluating the
code, eval_js() removes the object so the scripts from the loaded page
can't access it.

eval_js() supports returning a string result, and you can use it with
uzblctrl to query the DOM. Uzbl.run() also returns a string, but the
only useful thing you can do right now with that is to print @uri or
other variables.