Draft spec changes for comments

[mbays at sdf.org]

* Saturday, 2020-06-13 at 18:15 +0000 - solderpunk <solderpunk at SDF.ORG>:

>* Add a minimal definition of alt text to the definition of preformatted
>  mode toggle lines, to address accessability and searchability
>  concerns, as well as to close, or at least ameliorate, the
>  extensibility loophole identified by Katarina Eriksson.

* gemini://gemini.circumlunar.space/docs/specification-modified.gmi
> Any text following the leading "```" of a preformat toggle line which 
> toggles preformatted mode off MUST be ignored by clients.

This seems to be (partially) closing the loophole at the opening of 
a preformatted block, but leaving the matching loophole at the end 
intact? If it's ignored by today's clients, it can be used by 
tomorrow's.