Add proxy support.

[thibwk]

aiohttp doesn't trust the environment by default, so it doesn't use HTTPS_PROXY.

This will allow for a proxy to be set explicitly when instantiating the API.

[signalkraft]

Thanks for the PR, and sorry for the late response here.

I think it might best to extend the API __init__ with **kwargs and pass those along to the ClientSession, to support whatever other aiohttp features people may want to use. For example there's trust_env=True to read proxy settings from env vars.