Centralize searchers

All plugins with search capabilities expose their functionality now to the search plugin, which uses a directive ('pluginSearch') to pull in plugin specific templates. These templates need to be placed at a specific path (covention over configuration): `templates/arethusa.PLUGIN_NAME/search.html`.

We will allow more flexibility in the future.

For now all a plugin has to do is to define this template, and sets its property `canSearch` to true - once the search plugin itself is included in a configuration, all search functionality of individual plugins will magically appear.

Upcoming feature: Allow to chain searches - right now they are exclusive, i.e. you can't search for all tokens that start with a c and are singular.