Support no text item list

Markdown allows the case. For example:

    * Have text
    *
    * ^ has no text

GitHub: fix #37

Reported by Colin Dean. Thanks!!!
Add --initial-slide option

It shows the Nth slide at startup. It is helpful for debugging. :-)
theme: apply text size to preformatted block instead of preformatted texts
Improve drawing performance for non vertical centering mode

GitHub: fix #35

Reported by Colin Dean. Thanks!!!