init.el: Add useful keybindings.

Add keybinding for `toggle-fullscreen' and `auto-fill-mode'.
init.el: Automatically open .md files in md mode.
init.el: Improve function `get-cm-dates'.
Add prefix to all functions and variables in init.

Add the prefix 'init-' to all defuns and defvars in all three init files.
Improve formatting of some of them.  Adjust docstrings accordingly.
Correct function prefix 'init-' a few places.

Correct some references to the functions by their old name.
TODO: Update file.
Customize linum-mode's line formatting.

Make line formatting 0-padded but still only as wide as the number of the
last line in the buffer.