Rename Code: to Figure:

Check fenced code blocks and normal code blocks to ensure they
parse the Figure: string correctly.
Fix tests
Output crefs when the comment has user:comment

Otherwise don't output anything.
Output references at the end of the doc.

Reference output is ticked by {backmatter}, if we haven't seen that
tag, we fake it and output the appendix and references.
Prepare to move Figure: and Table: to after the object they describe
Figure: must be placed *after* the code.

Parsing it before it, becomes difficult, now it is just a paragraph
after the code, starting with 'Figure: '
Don't output matter when doc is not standalone
Table and Figure captions are now postfix.

Put them *after* the object you want to have the caption for. Just
like Table: in pandoc. But also allow this for figures.