allow frontmatter delimiters with trailing whitespace

[Alizter]

I had copy-pasted the theme setting that goes at the top from a pager which left a bunch of whitespace I hadn't seen.

---__________
theme: vanier
---__________

I was puzzled as to why it would display verbatim in the slides. I later worked out what the issue was. It might be useful to strip the whitespace after the --- or at least warn the user abuot it.

[panglesd]

Thanks for reporting! This needs fixing indeed.