Get `use-package' via package.el, drop submodule

MELPA now hosts `use-package'.
Move it to load-path.el so we can load and bootstrap package systems and require
`use-package' early in the initialization.
Some Twitter specific configuration

'twitter.el' is not checked in and in .gitignore
Source everything from (M)ELPA. Separate packages

Following the 'lisp' directory, also drop 'themes', installing "Tomorrow" theme
from MELPA.

Move all third party package descriptions done via `use-package' to a separate
file `packages.el' and also assign it a register to easier jumping to it when
necessary.
Simplify the way we enable some disabled commands

Drop the unnecessary `defun' and just `dolist' the list of disabled commands we
want back.
Hook `hl-line-mode' to `prog-mode-hook'

Highlighting the current row is useful for almost all programming language major
modes.  Some major modes still doesn't invoke `prog-mode-hook' but it's rather
new so eventually they'll start invoking this hook.  For now we'll add them to
`hook-into-modes' expressions, next to prog-mode-hook.