Add only enable when flying option

[HughBone]

This adds an option to only tilt the camera if the player is flying. The code is pretty sloppy but somehow it works.

[Mirsario]

💀Necro update almost 2 years later: some months after this PR, #18 added partial support for this in a more customizable way than a boolean is, allowing disabling of the strafe roll for non-flying or non-elytra cases, by setting their factors to zero. I assume that it's also wanted to have that for the forward velocity pitch modification, but that should be done similarly, and at that point all intensity factors should just be in 3 per-context objects.