added json output with tests

[oneyb]

Hi Samuel,

I wanted JSON output for ansible. Therefore, I added json output options. Hopefully this is helpful. I also included tests, but I am not familiar with pytest, and they failed.

For a quick view:

virtualenv -p python3 venv
source venv/bin/activate
pip install git+https://github.com/oneyb/syncthingmanager
stman folder info default -j
stman device list -j
stman folder list -j
stman folder list -h
pytest

It's more useful to me now. Thanks for the library whatever you decide :+1: Cheers, Brian