[RDY] pandoc filter to convert blockquotes to divs

This filter converts all blockquote-with-attributes to
div-with-attributes, as discussed in issue #4.

A blockquote will be converted if

1. it begins with a header
2. that header has attributes
3. those attributes contain a single class
4. that class is one of ['objectives', 'callout', 'challenge']

For example, this is a valid blockquote:

    > ## Callout time! {.callout}
    > Let's do something

and it will be converted into this markdown:

    <div class='callout'>
    ## Callout time!
    Let's do something.
    </div>

and the equivalent html.

I've integrated this into the `Makefile`, so `make preview` should show you
what it does.

TODO:

- [x] only allow attributes with one of the allowed classes

Things to consider:

- [x] Is the spec for a blockquote sufficiently tight / flexible?
- [x] Does the code do what it needs to / says?
- [x] Is this code maintainable? Does it need clearer documentation?
- [x] Do we pull in [pandocfilters] as a dependency?

[pandocfilters]: https://github.com/jgm/pandocfilters

We use triple tildes for code blocks (I know that triple backquotes work as well, but we're trying to be consistent).