have Settings modifiers controlled by .env

[marc-portier]

• avoid to have to add modifier --mode flag all the time
• read pysubyt.modifiers from environment
• support fallback to .env (include https://pypi.org/project/python-dotenv/)
• have this not (only) in the __main__() but in the core API so always available --> thus code should go into Settings.__init__() (in pysubyt/api.py) to call load_from_modifiers twice: first with the .env, then with the passed values
• will need assertion or over-ride in the test-cases though to make sure we have a fixed base for testing not messed up by some casual .env

[marc-portier]

while at it:
the --templates might make for another sensible candidate to be injected via .env