Customize key (ctrl+enter) to send message

[gitsang]

I need this feature because I'm used to using text editors where pressing enter creates a new line. The default behavior of sending messages with enter always makes me accidentally send messages which I haven't finished editing. Many chat tools also support replacing enter/ctrl+enter for creating new lines and sending. So I think this should be an option that can be added.

Additionally, because some terminals hijack ctrl+enter and turn it into a newline character, sometimes setting it to ctrl+enter may not work. If the terminal hijacking cannot be resolved, you can change it to a different key.

I have completed the code modifications. If I feel it's appropriate, I will create a PR.

https://github.com/gitsang/crush/commit/470f26ad157e2b2e9782d49115e4d48be3829fb8