Allow disabling SSL cert verification when using a proxy

[spenczar]

Fixes #129.

I had to write a helper function to get the right json encoding, since we need a JSON false for rejectUnauthorized.

I haven't written much lisp so let me know if the copilot--proxy-param is a horrible way to do this. It sure feels ugly.

[zerolfx]

How about updating the documentation and instructing users to add :rejectUnauthorized :json-false if needed? The only limitation is that this sub-option cannot be configured through the UI.

[spenczar]

Oh, that makes good sense to me. So users would just setq it themselves pretty explicitly - yeah, that seems much nicer. I will revise this when I get back to a computer in a few hours.

[spenczar]

This is ready for another look. Much nicer!