Add sprint speed multiplier for vertical movement

[BlakeBr0]

Adds an additional sprint speed multiplier stat for vertical movement while holding the sprint key.

• [x] Track users holding sprint
• [x] Add sprintSpeedVertMulti to jetpack json (defaulting to 1.0)
• [x] Apply sprint speed multiplier to vertical flight when sprinting

closes https://github.com/BlakeBr0/IronJetpacks/issues/4