markdown code parsing cleanup/tweaks

when adding code tags, now using {{markdown.cl|code|...}} tags. this
allows the escape-code-internals function to *only escape code inside of
a markdown.cl code block*. this is important so code blocks don't get
double-escaped.

also, making block splitting simpler. there's no need for it to do space
testing since anything that requires that is parsed BEFORE blocks are
split.
merging escape-code-internals into parse-entities, which does do-parse-entites *outside* code blocks, and escapes everything inside code blocks. this is working perfectly, so far...no double escapes, no entity mixups inside or outside code blocks. sounds good.