Add `session_kwargs` to `Connection`
Add `session_kwargs` to `Connection`

This allows to add arbitrary arguments to each `get` and `post` call. It
can be used to add proxy arguments:

    Connection(session_kwargs={'proxy': <your-proxy>})

See the aiohttp docs for details:
https://docs.aiohttp.org/en/stable/client_advanced.html#proxy-support

Supersedes and closes #49

lgrahl/threema-msgapi-sdk-python