filter out unplayable/unavailable tracks

[rileyallyn]

Resolves #158.
Resolves #211.

The purpose of this PR is to filter out unavailable songs. This functionality is achieved by utilizing Spotify's song relinking feature, specifically by sending the market query parameter and using from_token to retrieve the user's country code from their token.

A track is filtered out if its is_playable is false.

This did require a non significant refactor with the functions that call internal_call and the various internal pagination functions. But by making a "generic" payload field allows for more flexibility in the future if Spotify adds more query params.

[apprehensions]

Why not mark it as unplayable and have it still exist in the playlist under a 'grayed-out' color?

[aome510]

Why not mark it as unplayable and have it still exist in the playlist under a 'grayed-out' color?

To avoid possible conflict with other commands (for example PlayRandom). IMO, I don't see any point of showing such unplayable tracks, but you want it as a feature, it's not difficult to implement.