Refactored the old `--linear-volume` flag to a more generic `--volume-ctrl` flag that takes the options of `[linear, log, fixed]`. It defaults as previously to log.
* create Volume struct for use with Cache
* add "volume" file to Cache
* load cached volume on start, intial overrides cached overrides default
* amend volume_to_mixer function to cache the volume on every change
* pass cache to Spirc and SpircTask so volume_to_mixer has access
* rustfmt changes
* revert volume_to_mixer function and Spirc/SpircTask cache variable
* Volume implements Copy, pass by value instead of reference
* clamp volume to 100 if cached value exceeds limit
* convert Volume to u16 internally, use float and round to convert hex->dec
* convert initial_volume and ConnectConfig.volume to u16 as well
* add cache_volume function to SpircTask
* remove conversion to/from percentage on cached volume
* consolidate device.set_volume, mixer.set_volume, and caching
* streamline intial volume logic
