Add a persistence API to plugins

[dalloriam]

It'd be nice if the plugin host could periodically ask plugins to save their state & load back their state on app start via some plugin hooks.

We'd like a file-like API so that a plugin can save and load its state to a Box<dyn io::Write> when requested by the ShifTTT engine. From there, the engine could persist the state as it sees fit.

This would allow for e.g. the dir watch plugin to persist what files were seen to prevent rescans between instances or between boots.