(NOT READY) Add interactive version of with-editor

Hello,

I tried `with-editor` and it seems to work great.

Here is something (not ready yet) that I'd like to have in the final version, what do you think?

What still needs to be done is:

* Handle errors when saving (e.g invalid crontab fails silently atm)
* With prefix arg, ask which EDITOR variable to set
* Find a decent name for the buffer