More config goodness.

- separated global editor config completely from mode config
- edit-config -> edit-mode-config
- added edit-editor-config for editing global editor config
- added view-left, view-top to global editor config
Expose width/heightInChars.

I didn't end up using them, but let's leave them around.
Keep the minibuffer from spilling to the edges also.
Format our completions into columns, if possible.
Note re: } handling.
Turned completion fn into full-blown class.

This allows us to do two things:

1. Disallow entering something that's not a valid completion (useful when
   completing fns and things like that).

2. Provide a path separator so that we can strip off long shared prefixes when
   completing path-y things. This makes find-file way more pleasant.