Add composite commands

[sigmaSd]

Hello,

What do you think about adding this command:

enum Cmd {
Composite(Vec<Cmd>),
}

I needed this for the vi o/O commands, and I imagine other complex commands will need it.

Here is an example https://github.com/kkawakam/rustyline/compare/master...sigmaSd:clear_buffer?expand=1

[gwenn]

See #418, #306, #466, #342

[sigmaSd]

Closing then since the exact same issue was filed #306