Configurable plugins

@kalmiz This pull request moves the Closure template plugins into the play application's config file instead of being hard-coded. Due to limitations in how plugins can be configured in play, it uses reflection to turn a classpath (in the form of a string) into an instance of the class. I chose to swallow exceptions related to incorrect classpaths.

An example config section:
```
closureplugin {
    status = "enabled" # set to "disable" to turn off the closure plugin
    assetPath = "/var/tmp/kinja-mantle"
    plugins = [
        "com.google.template.soy.SoyModule", 
        "com.google.template.soy.xliffmsgplugin.XliffMsgPlugin",
         "com.kinja.soy.plugins.PluginsModule"]
}
```

I also copied our scalastyle/scalariform rules from mantle into the project. So you might want to review with ?w=1.