First slide must start with an H1

[timp]

To reproduce:

$ echo "# hi" > t.md
$ darkslide t.md
Adding 't.md' (markdown)
Generated file: presentation.html
$ echo "## hi" > t.md
$ darkslide t.md
Error: list index out of range
Adding 't.md' (markdown)

I was only using an h2 for formatting, so will fix formatting in h1.