Add a parameter to configure the focused-on bone

[Cwiiis]

Currently we focus the camera on the pelvis bone, which is a recent change from focusing on spine_02. Really, this should either be configurable, or we should have some options or behaviour relating to making sure the subjects are in view.

[pavel123]

Added --focus-bone parameter that allows you specifying the bone at which you want to focus camera on during rendering. pelvis is default.