markdown: just parse a language in code fences!

stop that special {} block syntax.
just take the first token (minus whitespace) and use that as language.
if line has more tokens, ignore them and return valid.
markdown: refactor codefence methods

prefix_codefence -> is_codefence
--- Used for checking presence of a valid codefence line

is_codefence -> (bool) parse_codefence
--- Expects single line, tries to parse it and extract language

Uses have been modified accordingly. Also, since
parse_codefence now requires single lines, the parse_fencedcode
method has been rewritten and simplified.
html: Remove PRETTIFY feature; instead output language classes
markdown: more precision on codefences

- is_codefence is now able to return width and character of the rule.
- parse_codefence accepts pointers passed to is_codefence
- parse_codefence now returns width of the codefence

- fenced code blocks MUST now be ended with the same character,
  width and lenght as the starting rule.
markdown: let parse_fencedcode put text if there's no end fence