Added previously-forgotten AutolinkFilter.

RSpec: 707 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Simple cleanup of MarkdownHtmlConverter spec.

RSpec: 707 examples, 0 failures
RuboCop: 179 files inspected, no offenses detected
Refactoring MarkdownHtmlConverter.

Isolating knowledge of RedCarpet(+Rouge) API in internal module, before
developing *parallel* module using new HTML::Pipeline code.

RSpec: 707 examples, 0 failures
RuboCop: 181 files inspected, no offenses detected