Warn on features that are dissabled at compile time

[ClashTheBunny]

I did a basic cargo install spotifyd and was confused about how mpris didn't work. Obviously I didn't add the feature even though I had the example config in place and taylored for my use case. It would be nice to have been warned about having those options enabled in the config without having them compiled in.

Describe the solution you'd like

• A warning when a config option is passed but ignored and why.
  and/or
• A note in the default config with what features are needed to get a specific option.

Describe alternatives you've considered
Expecting people to read the documentation :-D But really, I did eventually got there... The issues are hard to use because they all have mpris as a checkbox, which shows up in search even when not checked.

[GChalony]

+1 for this, I spent a moment debugging my setup before understanding that I had build spotifyd without MPRIS. I eventually noticed use_mpris = None in the logs rather than true as I would have excepted, which eventually made me realise the problem.

That's especially relevant since armv6's release doesn't pack MPRIS feature (I believe?)