Markdown parser should support embedding and restrictions on content

[jace]

1. Allowing externally hosted images creates an information leak wherein the external host receives traffic from users, revealing their IP, browser info and referrer. Some use cases may also require disallowing embedded images entirely.

2. Naked URLs on a paragraph by themselves should be converted into embeds where applicable (images, videos, tweets, etc)

Both require (a) additional flags to the Markdown parser, and (b) callbacks for custom processing, or custom extensions passed in.

[jace]

The MarkdownComposite type also needs to support these additional flags (a) in the column configuration and (b) in the instance method for one-off customization (for example, the Comment.message column in Funnel may change restrictions depending on the user's role).

[jace]

#184 is also for the MarkdownComposite customization requirement.