zsh,vim: Add WIP Zettelkasten tooling

Add `zk`, an autoloadable function that creates a new zettel after a
template in the right directory.
Add `zettel` vim ftplugin.

.config/vim/ftplugin/zettel.vim

@@ -0,0 +1,9 @@
+" Reuse git-commit textwidths for subject (minus the `# ` markdown
+" header-prefix) and body. This way the note (but especially the subject) should
+" be usable as a commit message.
+setlocal colorcolumn+=53
+setlocal textwidth=72
+" Spell checking always enabled
+setlocal spell spelllang=en
+" Automatic formatting
+setlocal formatoptions+=a