Remove Jekyll-specific directives from the markdown output

[rht]

From #2 (comment) :

% include is emitted into the markdown files which isn't Markdown -- I think we should avoid doing so in the official tool, since it presumes you're using something like Jekyll or a similar markdown renderer which is a superset of markdown.

[timabbott]

We should just make this an option controlled by the config file for whether the user wants Jekyll output, and document how that works.

[showell]

I am hoping to get to this after some substantial code cleanup that is happening in #19. That PR fixes the issue of include, but we also have complications related to "title". For example, if you use Jekyll, but don't have a layout yet (maybe just cause you haven't gotten that far yet), the title doesn't show up now.

[showell]

Sort of repeating my last comment, but we don't want to take any action on this until #19 gets merged.

In #19 I've made some structural changes that should make it a lot easier to support multiple targets. In particular, it isolates a lot of the Jekyll code in a pretty nice way.