Escaping in gemtext

[Ryan Westlund]

According to the Gemtext specification, any line that starts with
"```" is a preformatting toggle. This makes it impossible to have such
a line as part of a preformatted block. I understand the design goals
of Gemtext, but I believe the Markdown solution to the same problem
can be lifted straight into Gemtext relatively easily: allow more than
3 backticks to open a preformatted block, and require the same number
to close it as the number that opened it. This way, any possible text
can be included in a preformatted block. Thoughts?

The same issue exists with text lines (it's impossible to display "=>"
or "#" at the beginning of a line of text), but I'm not concerned
about that because having to prefix such lines with a space is not a
big deal for plain text, whereas it is for code.