Allow security policy to be configurable in the Custom Origin

[skyzyx]

I would love to be able to mandate TLS 1.2+ connections exclusively for my distribution. It would be wonderful if this were configurable via this plugin.

Note: This is different from #16.

See:

• https://github.com/Droplr/serverless-api-cloudfront/blob/master/resources.yml#L20
• OriginSSLProtocols