Increase Markdown flavor of Remarkup

Summary:
See https://github.com/facebook/phabricator/issues/82 and D1125. This makes
several minor changes to Remarkup:

• Adds a "`" syntax for code blocks. This comes from GitHub-flavored

Markdown rather than real Markdown, but is highly reasonable for web inputs
since there's no block indent. I always copy-paste into TextMate and use
Command-], but this is cumbersome in other editors and in general. The indent
syntax is still supported, as this is prettier and more convenient in some other
contexts (writing documentation, for instance).

• Adds forward-binding for code blocks to support ` syntax and lay the final

edge case from D1125 to rest.

• Adds "*" support for lists, similar to Markdown.
• Drops the requirement that lists be indented. I believe this was an artifact

of a very old, less-powerful parser.

I'll follow up with a doc update, too.

Test Plan: Ran unit tests.

Reviewers: btrahan, broofa, fugalh, jungejason, nh, aran

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: 1148