Update _buku

Added support for markdown option

auto-completion/zsh/_buku

@@ -20,6 +20,7 @@ args=(
     '(-l --lock)'{-l,--lock}'[encrypt database]'
     '(--merge)--merge[merge another buku database]:buku db file'
     '(--noprompt)--noprompt[noninteractive mode]'
+    '(--markdown)--markdown[markdown mode]'    
     '(-o --open)'{-o,--open}'[open bookmark in browser]:bookmark index'
     '(-p --print)'{-p,--print}'[show bookmark details]'
     '(-r --replace)'{-r,--replace}'[replace a tag]:tag to replace'