Add plugin support

[egg82]

This release finally adds support for custom plugins. This includes a new plugin for webhooks as well.
More information on creating and using plugins in the README.

Plugin quickstart:

To try the example webhook plugin, create a file called plugins.yaml in the Fetcharr config directory with these contents:

plugins:
  - url: https://repo.egg82.me/releases/me/egg82/fetcharr-webhook/1.0.0/fetcharr-webhook-1.0.0.jar
    filename: fetcharr-webhook.jar

Then, restart Fetcharr to download and start the plugin. The webhook plugin will create a configuration file at <config dir>/plugin/webhook/config.yaml. Configure the plugin and restart Fetcharr again to get the plugin to pick up the new values.

This new version also makes use of the data directory, though only for plugins that would need to store long-term data. Currently no such plugins exist.