Adding options to allow disk outputs to always be in a specific unit.

[SWarrener]

This PR allows the user to force the disk output to always be in a specific unit, and not automatically change unit as the size in question changes. See below for a few example configs, which should demonstrate this change.

"disk": {
	"interval": 30,
	"format": "{specific_free:0.2f} GB out of {specific_total:0.2f} GB available. Alternatively {free} out of {total} available",
	"unit": "GB"
	// 1434.25 GB out of 2000.00 GB available. Alternatively 1.4TiB out of 1.9TiB available.
}

"disk": {
	"interval": 30,
	"format": "{specific_free:0.2f} GB out of {specific_total:0.2f} GB available. Alternatively {free} out of {total} available",
	"unit": "GB"
	// 0.25 GB out of 2000.00 GB available. Alternatively 241.3MiB out of 1.9TiB available.
}

I have added new options instead of updating existing ones to ensure I don't change anyone's existing config.
Supported specific units are: B, kB, kiB, MB, MiB, GB, GiB, TB, TiB