Test: Improved Markdown grammar

[chrmarti]

Test for Markdown September issues

• Any OS @joaomoreno

Complexity: 1

• Verify test.md markdown file colorizes properly.
• Verify http://daringfireball.net/projects/markdown/syntax.text colorizes properly
• Verify vscode README colorizes properly.
• Verify files from markdown test suites colorize properly. Example set; https://github.com/wkpark/markdown-testsuite/tree/master/Tests
• Test that any filetype opened as markdown can be previewed using the markdown editor.