wscat: Support sending control frames (ping, pong, close)

Commands are prefixed with a slash "/" character.  The remainder of the
input line is used as command parameters. Also, print out ping and pong
control frames when they are received.

Example:

    $ ./bin/wscat -c ws://echo.websocket.org
    connected (press CTRL+C to quit)
    > hi there
    < hi there
    > /ping
    pong
    > /ping echo
    pong echo
    > /close 1001 bye
    disconnected