Allow (swallow) leading blank space

[cabo]

The first line of a kramdown-rfc input file decides whether kramdown-rfc should work on a snippet only, whether we are looking at a complete kramdown-rfc document ("---" in first line, followed by YAML Metadata), or or whether we actually are looking at mmark input ("%%%" in first line).

Be more robust against this being obscured by leading blank space.

(Solution is not completely trivial because of line number management for error messages.)