[draft-js-export-markdown] Support code fence

[sstur]

This adds support for an option to export Github-style fenced code blocks.

You can pass a second (optional) argument like so:

stateToMarkdown(contentState, options)

The only option currently supported is:

{gfm: true}

The test-cases.txt file as been updated to allow passing options after the description (separated by a "|").

This PR addresses withspectrum/spectrum#4472.

[mxstbr]

Maybe calling the option gfm: true as it is called in draft-js-import-markdown would make sense? Other than that, this looks perfect!

[sstur]

OK, done. I've made that change.

The reason for calling it codeFence is because I'm not sure what other GFM features there are and which ones we might want to support in the future and if we'd want to give them separate options (meaning if you're using gfm: true and later we add another gfm feature under that option, you, as the consumer, might one day get some changes to the way your stuff works without realizing it.)

That being said, I'm going to land this with your suggestion and yolo 😄