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.
Merge with upstream.

Conflicts:
	core/prelude-packages.el
          - Added ace-jump-mode to the list of packages.