Add ssl_cert_store option for custom CA certificate store.

I want to specify all the SSL options using strings (coming from a database) instead of file paths. excon already has the `certificate` option for the client certificate and `private_key` for the client private key, but it doesn't seem to have an option for passing in a CA certificate.

After reading the ssl_socket code, I figured that it'd be better to add an option for specifying a custom `OpenSSL::X509::Store`. My code can create a store and add a certificate to it, just like in the test in this PR.

I look forward to your feedback. I hope I can address any issue that comes up, and hope to get this functionality merged.

Thank you very much!