Adds configuration for customizing SSL connections

[legrego]

This introduces two new optional configuration settings to control the behavior of SSL connections:

elastic:
  verify_ssl: true # set to false to disable SSL certificate verification
  ssl_ca_path: /path/to/ca.crt # fully qualified path to custom pem encoded certificate authority

Fixes #33

@jakommo I tested these configuration options locally, but if you get some time, I'd love to get your input here to make sure I captured your requirements.

[jakommo]

Tested both options and they both work for me.
Thanks for the quick implementation!