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/zsh/.zprofile

@@ -40,6 +40,8 @@ if [[ $OSTYPE =~ darwin && ! $PATH =~ "/Library/Apple/usr/bin" ]]; then
 	export PATH="${PATH:+$PATH:}/Library/Apple/usr/bin"
 fi
 
+export ZETTELKASTEN_NOTES="$HOME/docs"
+
 # Locale settings as $LANG
 [[ ! -e "$XDG_CONFIG_HOME/locale.conf" ]] || . "$XDG_CONFIG_HOME/locale.conf"