merge cli and config flags

[mainrs]

This PR merges the CLI and config options into one common struct to prevent different naming schemes. As far as I can tell, the change is fully backwards compatible thanks to clap's and serde's alias keywords.

I basically only changed the parsing logic, the outer interface is still the same (SpotifydConfig).

CLI arguments take priority over config arguments. They are merged together during load. In theory, different file formats other than INI could be supported but I opted out of this one for now. I had to re-declare librespot enums like Bitrate to add help text to them.

The following keys have changed (the old keys could be removed in a breaking change later on):

Old value	New Value
volume-control	volume_controller
volume-normalisation	volume_normalisation
use-keyring	use_keyring
normalisation-pregain	normalisation_pregain
onevent	on-song-change-hook

The idea behind the last one was to add more unified hooks later on (like on-login-hook or on-device-connected-hook, meaning that people can create more custom scripts to show notifications or do other stuff).

I still need to clean up the code as I can re-arrange a lot of stuff to make it more readable.
Creating the PR so you guys know that someone is working on it :)

The following things still need to be done:

• cleanup dependencies
• re-format code
• re-structure cli and config structs (embedding them the other way around makes more sense)
• implement zeroconf and http

The PR will, after completion, affect the following issues:
Closes #233.
Closes #265.

[mainrs]

I think I got everything done. Would be nice if you could take a look and give an OK @simonpersson :)
The flags and the config entries should be completely backwards compatible.

Ups, just noticed that I actually do not check for the spotifyd and global headers in the config file 🤐

[SimonTeixidor]

Thank you for this! I have been away from my computer for some time, so I wasn't able to review it for you. Looks like a great improvement :)