with-editor review

Hello,

As usual it's likely that more commits will follow based on the discussion.

1. What do you think about this commit (moving eshell in "right" place)?
2. I added a todo item in #113, namely the one about `eshell-preoutput-filter-functions` which is never cleaned. `eshell` has an exit hook that we could use to cleanup, what do you think? Should we care?
3. All the "invisible" commands TODO items are not really solvable without some filtering pain, which makes me think it's much easier to add a `echo with-editor ready for EDITOR` at the end of what we insert in the buffer. Possibly with a `clear` before so it looks neat. What do you think?