Ignore everything except some explicitly listed files

Different packages may place files into the emacs-directory. I think it is safe
to assume that this repo won't have a frequent need for other files, so let's
just ignore everything but the files that we know is going to be around forever.

.gitignore

@@ -1,11 +1,5 @@
-.mc-lists.el
+*
-auto-save-list/
+!LICENSE.md
-autosaves
+!README.org
-custom.el
+!init.el
-eln-cache/
+!init.org
-elpa
-init.elc
-projectile-bookmarks.eld
-recentf
-smex-items
-transient/