audio_cache limit & auto-management.

[SeasonalFerret]

Please tick all applicable boxes.

• [x] I am using Python 3.5 or higher (run python --version on the command line)
• [x] I have followed the official guides to install the bot for my system
• [x] I have updated my dependencies to the latest version using the appropriate update script

Which version are you using?

• [x] The latest master version (1.9.7)
• [ ] The latest review version

What type of issue are you creating?

• [ ] Bug
• [x] Feature request
• [ ] Question

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Description of issue

Set a storage limit for audio_cache. I don't have a 20TB harddrive, so limiting MusicBot to 1-2GB would be nice. A method for choosing what it should delete could be based off of what songs are worthless to keep.

Example: After audio_cache reaches (X)GB of data, go through and delete songs that either haven't been played in the past (X) weeks or that have only been played once or twice.

This would keep the audio_cache folder limited to only popular songs.

An exception may be songs on the autoplaylist.