Add a offset to throttle

As motor command must be something between 1000-2000 and
controller send something between 0-1000, add this offset.
Add option to config esc

This way we can calibrate PPM range and
also configure battery type and enable/disable brake.

REMEMBER: most of ESC enter in configuration mode when
the first PPM pulse is in the maximum value. This way
motors will not arm and start spinning.

Look at your esc documentation to know how proced configuration.