I'm not familiar enough with this Gem but I'm guessing that {} used to
work in place of 'Hash.' It doesn't anymore, and this wasted a couple
minutes for me, so I'm fixing it here. I could be wrong though?

Markdown has to first output a <pre> with a "lang" attribute for syntax
highlighting filter to be able to pick it up.

Removed declarative code in favor of a validation hook. It's up to subclasses
to implement the validate method. Validate will be called by Filter after
initialization so @context is available.

Reverted README.md to remove the additions about validation.

Noted in filters if a context is required. Updated README to include info about
required contexts.

Noted in filters if a context is required. Updated README to include info about
required contexts.

This is useful if an image is root relative but should really be going through
a cdn, or if the content for the page assumes the host is known i.e. scraped
webpages and some RSS feeds.