Refactor Volume control, allow for a fixed volume option

[ashthespy]

This is a (long) overdue follow up to #286 (comment).

I have refactored the old --linear-volume flag to a more generic --volume-ctrl option that takes the options of [linear, log, fixed]. It defaults as previously to log.

What fixed does is disable the client side volume control, by setting kVolumeSteps to 0, so no volume events are passed to the client.

As usual, this will need a update of the wiki, and should be considered a "breaking change" when considering the versioning.

Fixes #48, Closes #286

[sashahilton00]

Thanks for this, will give it a test. W.R.T the breaking aspect, we might have to hold off merging this for a bit. We might be best off considering this for the v1 release, along with #134. Also, at some point we need to start thinking about fleshing out librespotd, and stabilising librespot as the core library. That way changes such as these can be added to the former without breaking the latter.

[ashthespy]

@sashahilton00 just to clarify the "breaking" change here would be the flag passed into the binary, and the VolumeCtrl enum that libraries using spirc will need.
Not sure if this would be considered a version bump, and we are still merging to the dev branch anyway..

[sashahilton00]

This can be reviewed and merged to dev now that v0.1.2 is released.

[sashahilton00]

@ashthespy since the docs are broken and shouldn't really be, I think we can just merge this into dev along with #504 (assuming this is actually a fix) and release it as v0.1.3 - let's face it, we're not exactly rigorously sticking to semver atm ;)

[ashthespy]

Sounds like a plan, I'll fix the conflicts later today.

[ashthespy]

Rebased on dev, and fixed the conflicts. I was a bit trigger happy with the merge, but it should be all good now. Wiki has also been updated.