Improve mdvimb() to accept no args or `-` for stdin

dotfiles/bash/main.bashrc

     #
     # Open Markdown file in Vimb (no junk)
     #
-    local file=$1
-    <"$file" Markdown | vimb -
+    local file=$1; shift
+    case $file in
+        "") Markdown | vimb - "$@" ;;
+        -)  Markdown | vimb - "$@" ;;
+        *)  Markdown < "$file" | vimb - "$@"
+    esac
 }
 
 nlfor() {