src: Select the data inserted by ! and <a-!>

[lenormf]

Closes #1468

[aver-d]

I've tested this patch for a few months. It works nicely, thanks.

The main advantage is being able to immediately press s to select inside the newly inserted text, and it's also useful for moving the entire text for alignment or indentation.

[aver-d]

I forgot to mention that I've noticed that when the command returns no output or has an error (eg. ! printf <ret> or !unknown-cmd <ret> ), then the selection extends upwards towards the first character in the buffer.

I typically use ! programmatically inside scripts, so this has not come up much for me, but I suppose in the case of an error I'd expect no change in the current selection. Maybe an error message would be useful too.

[lenormf]

I think I see where the problem is, I'll look into it.

[lenormf]

I fixed the issue and rebased on master.

[lenormf]

Updated the documentation.

[lenormf]

Rebased on master.