[SPECS text/gemini] Heading lines proposal

[Solderpunk]

Okay, I am happy with this idea (in fact, for a minute or two I was
thinking "didn't we already do this?!" - no, that was with list items,
to avoid ambiguity with *this* kind of emphasis).

At this point it seems like we may as well go "whole hog" and insist on
at least one space after *all* the line type markers - that would mean
quote lines starting with "> " and link lines starting with "=> ".  Does
anybody object to *that*?  I strongly suspect that almost everybody is
actually writing their content that way anyway, but perhaps not...

Cheers,
Solderpunk