SSL endpoint commands

I think this is a good start for SSL functionality in hk. /cc @max @brandur @friism

```
➜  ~ hk ssl
Hostname:        akita-1234.herokussl.com
Common Name(s):  bgentry.io, www.bgentry.io
Expires:         2016-01-02T23:59:59Z

➜  ~ hk ssl-add-cert cert.pem key.pem  
Added cert for myapp at miyagi-7234.herokussl.com.

➜  ~ hk ssl-destroy
warning: This will destroy the SSL endpoint on myapp. Please type "myapp" to continue:
> myapp
Destroyed SSL endpoint on myapp.

➜  ~ hk ssl-rollback-cert
Rolled back cert for myapp.
```