Makes the localization variable configurable
Adds localizerVariable to Manifest.

* If present the localizerVariable will be used as the slug that the
  options are written to.
Makes OptionsManifest use the plugin slug as the localizerVariable.

* The localizing javascript variable will now be written as the
  underscorized version of the plugin slug.
Updates changelog.
Makes the javascript variable that the plugin writes configurable.

* The options variable now will correspond to the underscorized plugin
  slug name.