* ConversionTest.fs: Implementation for markdown links, along with
  unit test

* HtmlToMarkdown.fs, MarkdownContainer.fs, MarkdownLink.fs:
More complex tests that mix several types of nodes

These tests were added to both the dom, and parse test harnesses.
Refactor towards functional-ness

* ConversionTest.fs: Some of the helper methods I was using in the
  conversion tests were causing side effects ... namely, accepted a
  container parameter, and added the resulting node to the list of
  children. This means that every time you call the function, it
  changes the state of one of the parameters, making it
  non-idempotent. The refactor omits the container
parameter, and returns the object being constructed ... leaving it up
  to the caller to do what they want with it.


* MarkdownContainer.fs: The other change was to change the
  `MarkdownContainer`'s `add` function to return unit, rather than
  itself. Although
the original intent with returning `this` was to allow a chaining
  mechanism when you were adding multiple nested nodes,
it resulted in always having to terminate the chain of calls with ` |>
  ignore`. This was just plain ugly, and the tradeoff was
not worth it.