allowing the user to specify desired cipher suites to be used in the client-proxy interface

This is a (partial) reintroduction of a feature available in mitmproxy 0.8, and depends on a similar pull-request made to netlib.
The feature is accessible via the --client-ciphers option and its use is similar to the one in 0.8

Unfortunately, I didn't find a way to automatically test this feature, as pyOpenSSL doesn't seem to have a wrapper for OpenSSL's SSL_get_current_cipher, and get_cipher_suites only returns the available cipher suites, not the one chosen for a session. I tested it manually using Wireshark and there is a partial test in netlib.