Use 'custom_sorting' in formats dict to influence prefer-free-formats sorting

My suggestions for d80044c235afcbcfa905ab08a82f6f972df25268
With d80044c235afcbcfa905ab08a82f6f972df25268 3D and Dash videos are still preferred when prefer-free-formats is used.

Documentation in ```extractor/common.py``` is missing.