shell-command support

Hello,

Don't pull this yet as this was rebased on my `next` branch which includes the changes done in #96.
Once #96 is merged I'll rebase this branch on `next` instead.

I reworded your commit from `wip` to `Advice shell-command`.

With these changes I was able to do `M-& crontab -e` and `M-& git rebase -i HEAD^4` and both worked.