Parse and render practices using the MDX components

[NathanLovato]

Depends on #65

This:

<Practice>

<Requirement>you need X</requirement>
Hey, this is some _text_
<Requirement>you need Y</requirement>
<Hint>do the thing</Hint>
<Hint>do the other thing</Hint>

<Description>The description</Description>

<Note>blah, a note</Note>

</Pratice>
<Practice>

Another practice

</Practice>

Should become this:

<div class="practices-wrapper">

  <section class="practice">
    <p class="practice-description">
    The description
    </p>
    <ul class="requirements">
      <li>You need X</li>
      <li>You need Y</li>
    </ul>
    <p>Hey, this is some text</p>
    <div class="note">blah, a note</div>

    <ul class="hints">
      <li>Do the thing</li>
      <li>Do the other thing</li>
    </ul>
  </section>

  <section class="practice">
    <p>Another practice</p>
  </section>

</div>