ark-cli: Server mode and cache pre-generation

[kirillt]

Would be cool to have a subcommand serve which monitors list of roots and generates various cache for every root. This would help low-power devices to obtain the cache through the network instead of generating it on the device. As soon as new file synced to the server instance, ark-cli would generate preview, thumbnail and maybe some another kind of cache in the future.

The cache is replicated and persistent, meaning, it is wired through Syncthing to other devices (later we will implement our own syncing mechanism).