Don't clear screen after command completion

[a-peyrard]

I haven't find any option to do that. And quite often I'm using yank and I want to keep the output of the piped command in my terminal.

This PR, adds a new flag: X. When this flag is used the output is not
cleared after yank is completing.

[mptre]

Hi, I'm a bit opposed of merging new features at this point unless there's a clear demand from many. So I'm going to pass for now. However, the diff looks good.

[a-peyrard]

Ok, no problem, I can totally understand that.
I will use a forked version, hoping that one day, this feature can make it.

By the way I find yank amazingly helpful, it's definitely a good addition to my tool belt.