lines of three or more tildes.)   This involved some optimization of
the blockification code in markdown.c -- I now cache line type in the
line and have a cache filling function (`checkline()`) that will compute
the line type _once_ and then all the lookup functions can use this cache.
It's not significantly faster than the old code (1/100th of a cpu second
faster processing 10 450k documents) but it's also doing the line check for
fenced code blocks.