Client plugins

Support for loading additional scripts on frontend.
Added basic hooks for plugins to client script.

Scripts can be added to ~/.shout/plugins directory.

Registering a plugin:

    shoutPlugins.push({
        onmessage: function(message) { console.log(message); },
        onprerender: function(name, data) { console.log(data); },
        onppostrender: function(name, html) { console.log(html); }
    })