Add setting for disabling the plugin

The plugin can be totally skipped by setting `config.proxy.enabled` to `false` or empty string (`""`). This can be useful to for example disable it for some provider.

This also a step closer to auto-detection (#38), but I found it better to make separate pull requests of them. Especially as I haven't decided all the details about the latter yet. ;)

Reviews, testing, comments, etc. welcome as always.