[Fix #444] Add command prelude-update-packages
New keychord xx -> execute-extended-command
[Fix #447] Extract ido config into a module

This should make it simpler to disable ido completely if you want to use
an alternative library (like `icomplete` or `icicles` for instance).
[Fix #451] Add the ability to preload personal code
Fix typo in prelude-ido
Merge pull request #452 from jjcomer/master

Fix typo in prelude-ido
Add prefix arg support prelude-update-packages

With a prefix arg it will update all installed packages.
Merge branch 'master' of github.com:bbatsov/prelude
Fix epl-upgrade invocation
Merge branch 'master' of https://github.com/bbatsov/prelude

* 'master' of https://github.com/bbatsov/prelude:
  Fix epl-upgrade invocation
  Add prefix arg support prelude-update-packages
  Fix typo in prelude-ido
  [Fix #451] Add the ability to preload personal code
  [Fix #447] Extract ido config into a module
  New keychord xx -> execute-extended-command
  [Fix #444] Add command prelude-update-packages