Goto map

DO NOT MERGE

This implements a new keymap, currently called `goto-map`, for moving
between different major-modes. This is inspired on the websites `g`
hotkey. You can `g m` to go between meta and main site, for instance

The problem is that I don't know which key to use for this keymap. We
can't use `g`, that already stands for `refresh` (which is pretty
widespread in Emacs).

I thought of `j`, for jump, but that's also taken.