First attempt at hyperlinking mentions inline (see #877).

I'm doing the hyperlinking by parsing the generated Markdown HTML
into a DOM tree using Nokogiri, and then modifying the DOM tree in-place
by finding text nodes containing mentions, ignoring everything in
code blocks. The DOM is then re-serialized to HTML and returned by the
Markdown renderer in `lib/exercism/markdown.rb`.

This is the only way I know of doing the hyperlinking that both gives
us control over where the hyperlinking occurs, as well as preventing
XSS vulnerabilities from creeping into our code. I also looked into the
Redcarpet API, but it didn't seem to give us the same flexibility.

Because of the extra re-serialization that occurs during the post-processing
of the rendered Markdown, HTML output isn't *exactly* the same as it
was before, though it still has the same basic DOM structure. See the
change in the expected output of the `test_newlines` test in `MarkdownTest`
for an example of this.

I'm guessing that since the rendering now effectively enumerates all
the mentions, we might want to have it return some kind of object
containing both the rendered HTML as well as the list of mentions,
obviating the need for the `ExtractsMentionsFromMarkdown` class.

Things that still need to be done:

  * Right now we don't ensure that there are only spaces/punctuation
    around the mention, meaning e.g. our code would think `hi@toolness.com`
    contains a mention of `@toolness`. However, I *think* the
    `ExtractsMentionsFromMarkdown` class has the same limitation
    (not sure though).
  * A class `mention` is given to the hyperlink but it's not actually
    given any special display via CSS.
  * The list of mentions are still enumerated after the comment, but
    should be removed as per the discussion in #877.
  * Probably other stuff I'm forgetting about!

Also, as I am a complete Ruby noob, the code I've added here is almost
certainly terrible.
Merge pull request #941 from toolness/hyperlink-mentions-inline-rebased

Rebased attempt at hyperlinking mentions inline.