Add markdown formatter

[joe-sharp]

Add a new formatter that outputs markdown. I copied the style from the existing HTML formatter so that after processing, they should be the same looking to the end user. I also added tests for this new formatter.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

What's your use case for such a formatter?

[joe-sharp]

What's your use case for such a formatter?

I made it to be able to add comments to GitHub PRs in a nice format. An additional use case could be for use with static site generators such as Jekyll.

[bbatsov]

I copied the style from the existing HTML formatter

I still made a couple of suggestions that I think would improve the readability of the generated output. You'll also have to mention this new formatter in the docs.

[bbatsov]

FYI - I'll be cutting a new release some time tomorrow, so we'll have to wrap this up today if you want it to make the release.

[joe-sharp]

@bbatsov thank you for the heads up about the release. I am not in a rush for this, but I do hope to pick up work on this today. Apologies for the delay!

[joe-sharp]

Thanks much for these suggestions, I think the report looks quite a bit nicer and definitely more readable!

[bbatsov]

Thanks!