Fix markdown.js codepane bug.

[sjstyle]

This pull request fixes CodePane render bug when using markdown syntax.

Sample presentation code

Note : I attached Presentation class without import clause. You can paste and run this code at example/index.js.

export default class Presentation extends React.Component {
  render() {
    return (
      <Deck transition={["zoom", "slide"]} theme={theme} transitionDuration={500}>
        <Slide transition={["zoom"]} bgColor="white">
          <Markdown>
              {`
  ## CodePane
---

  \`\`\`
  console.log("Hello world!");
  \`\`\`
            `}
          </Markdown>
        </Slide>
      </Deck>
    );
  }
}

At current master branch )

After fixes branch )

This happens because CodePane needs source with props.source or children, but marked.js (markdown renderer) returns code property to implement them.

I fix additional function by changing property code to source, so we don't modify CodePane nor marked.js source.

[mAAdhaTTah]

This should fix #329.

[kenwheeler]

Can you rebase this to use the latest markdown style on master?

[mAAdhaTTah]

@kenwheeler @sjstyle I integrated the above changes + changes to Code for the Markdown component but I'm getting a new error in my code. See here: mAAdhaTTah@a71765c

Edit: Actually, this may be unrelated.

[sjstyle]

I noticed that this issue was solved by other commits. So I will close this pull request.

[sjstyle]

@mAAdhaTTah @kenwheeler
I opened another pull request #338, which relates 329 issue.