For
```foo
asdf
jkl;
```bar
we would previously generate
<pre>foo
f
;
</pre>
```bar
Now we generate
<pre>
asdf
jkl;
</pre>
* no longer cut off the first 3 characters on most lines.
* don't include the closing ``` line in the output. in addition to
omitting the '```', this also honors "Any text following the leading
"```" of a preformat toggle line which toggles preformatted mode off
MUST be ignored by clients." from the spec
* ignore the alt text after the toggle-on text. the spec leaves it to
the client what to do with that alt text, but it tends to be metadata,
and omitting it is simplest for the implementation, so do that.
Improves ascii art on many gemini pages, e.g. gemini://carcosa.net/
