Markdown lint

[jelbourn]

I would like a markdown lint for the components repo, and we probably want this for other repos as well. Bonus points for a markdown formatter as good as (or close to) the one we have inside Google.

Some lints I care about:

• Using the correct headers (especially for integration with our docs site)
• Code fences have a valid language
• Max line length (100)
• Certain HTML tags (e.g. mat-, cdk-) used in paragraphs without code formatting