Add function to open root index file

Call up `require 'zettelkasten'.open_index()` to open the file.
Pass along a file name `require 'zettelkasten'.open_index('home')` to
open said file at the zettel root instead.

lua/zettelkasten/action.lua

@@ -52,4 +52,19 @@ function A.make_link(visual)
                                                                  start_col))
 end
 
-return {open = A.open, open_selected = A.open_selected, make_link = A.make_link}
+-- Opens an index.md file at the zettelkasten root directory whether it
+-- exists or not.
+function A.open_index_file(name)
+    local link = {
+        ref = o.zettel().rootdir .. "/" .. (name or "index") ..
+            o.zettel().extension
+    }
+    A.open(link)
+end
+
+return {
+    open = A.open,
+    open_selected = A.open_selected,
+    open_index_file = A.open_index_file,
+    make_link = A.make_link
+}

lua/zettelkasten/init.lua

@@ -28,10 +28,15 @@ function ZK.open_or_make_link(visual)
     if not ZK.open_link() then ZK.make_link(visual) end
 end
 
+-- Open index file at zettel root directory. If title is passed in opens
+-- `title`.<extension> file, otherwise defaults to `index`.<extension>.
+function ZK.open_index(title) return action.open_index_file(title) end
+
 return {
     get_zettel_list = ZK.get_zettel_list,
     get_anchor = ZK.get_anchor,
     open_link = ZK.open_link,
     make_link = ZK.make_link,
-    open_or_make_link = ZK.open_or_make_link
+    open_or_make_link = ZK.open_or_make_link,
+    open_index = ZK.open_index
 }