Add ace-jump-mode to the default package set.
Move Prelude's menu item under Tools.
Add imenu support for markdown-mode.
Fix the toggle behavior of prelude-mode.
Make the list of modules loaded by Prelude configurable.

Modules loaded by Prelude can now easily be adjusted via
the file `prelude-modules.el` in Prelude's installation folder.
To disable a module simply comment it out; conversely uncomment
a module to enable it.
Auto-install slim-mode.
Map Command to Meta as well.
Remove dubious workaround for package.el timeout problem.
Fix #255.

`prelude-duplicate-current-line-or-region` was broken after the introduction of
dash.el.