Add some image tests from old parser.
Add a couple tests for character entities.
Add my `code`[lang] extension, tests for that and other inline code.
Add another regression test I found in main.rkt.
Fix bug with $char-entity: Produce a number not a char.

xexprs use numbers not chars for these.
Kill whitespace.
parse-markdown takes symbol? not string? for footnote prefix.
Provide with contracts in preparation for finalizing switch-over.
Rename main.rkt to old-main.rkt.
Add new main.rkt
Add regression test for issue #22.

Althoguh this was already fixed in the Parsac-based redesign, it's a
good test case to add.