Ability to connect to an endpoint that is protected by HTTP authentication and/or SSL Client Certificates

[eta-orionis]

Basic idea: pass a configuration file as a new, optional argument. The config file specifies HTTP authentication credentials and/or SSL/TLS authentication certificates. Syntax: YAML

If the config file has been given and there is a client certificate specified in it, then in collector.go a http client with added TLS configuration will be used instead of the http.defaultClient.

If HTTP authentication credentials have been specified, the request will be updated to include these.

From there on everything works as usual/was not changed.

I also added everything necessary to test this: TLS certificates for both server and client, provided an example configuration, updated the nginx config (copy-pasted existing block, added certificate directives), updated the README, and added a README for how to generate certificates so people can generate their own in case of need.