Add optional Symphonia backend

[aschey]

Resolves #351, Resolves #256

This adds the option to use Symphonia as an alternative backend.

• Symphonia decoders are all disabled by default and can be enabled using the symphonia-{codec name} features.
• Symphonia decoders and the default decoders are mutually exclusive so the user must set default-features = false to enable any of the Symphonia features.
• Added symphonia-all feature so the user doesn't have to enable each codec separately.
• Added a compile error that should clearly explain the issue when both implementations of the same codec are enabled.
• Updated the CI script to run against both the Symphonia codecs and the default codecs.
• Updated the docs to explain how to use these features.

[est31]

Thanks!

[Shnatsel]

@pdeljanov FYI - you've got users!