[FEATURE] Add motion state detection

[tigattack]

A feature I'd love to see in Owntracks is detection & sharing of "motion states", something that's implemented in the Overland app for iOS.

Overland's documentation describes it like so:

motion - an array of motion states detected by the motion coprocessor. Possible values are: driving, walking, running, cycling, stationary. A common combination is driving and stationary when the phone is resting on the dashboard of a moving car.

I use Owntracks on my phone to send data to my own location tracking/history service, and it'd be really useful to have this data.

Let me know what you think.