Initial support for user configurable SSL ciphers

Adds user configuration for the following Erlang SSL settings:

* secure_renegotiate (default = false)
* ciphers (default = whatever that version of Erlang decides)

This is useful immediately because Erlang less than R16B03-1 has incorrectly implements handshaking with elliptic curves, causing hard to debug SSL errors in recent versions of Chrome, Safari, Firefox, curl and go (in a very incomplete list).

It also allow administrators to change the priorities of ciphers for their installation or remove ciphers in the wake of attacks.