Correct Slack Markdown code block variant

[ellatrix]

Description

This corrects pasting code blocks from Slack. Unfortunately they insist on another Markdown variant, so I see no other way of fixing the code block right before we call showdown.

This is `inline code`.
```This is a code block.```

Adds tests.

Also adds showdown option to not add extra line break to code blocks.

How Has This Been Tested?

Copy some text with a code block from Slack. Before it would put the code inline. Now it should create a code block.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[ellatrix]

Also added the simpleLineBreaks option to preserve line breaks. This is very useful outside Slack paste too.

[mcsf]

Nice. :)

[ellatrix]

Thanks!