Lint markdown

[alexilyaev]

We should use markdownlint-cli, but they don't seem to support glob patterns so we can't ignore node_modules (opened a ticket).

Another option is Gil's check-all-markdown, but it has no flag to run just the Markdown lint without the URL checks (opened a ticket)

Or we can write our own package to just lint Markdown with markdownlint-cli using globby to support normal glob matching and ignore node_modules and .git folder by default.

[alexilyaev]

Looks like markdownlint-cli added the flag:
igorshubovych/markdownlint-cli#16

[alexilyaev]

Something else I came across:
https://github.com/remarkjs/remark-validate-links

[alexilyaev]

Also linting dead links:
https://github.com/textlint-rule/textlint-rule-no-dead-link
Part of:
https://github.com/textlint/textlint