Add ability to watch files in a folder

[ch1ller0]

Problem

Revalidating playlist on each drain comes with drawbacks

• It takes way more time than necessary and blocks the event loop, even though it has already scanned through most of the tracks
• You have to wait for playlist revlidation before using the newly added track\

Solution

Need some mechanism for revalidating the playlist on-the-fly (maybe use chokidar)