stylelint: Add font-family-no-missing-generic

Describe the issue. Is it a bug or a feature request (new rule, new option, etc.)?

This is a proposal for a new rule that always requires a generic font-family at the end of font-family list. Helps reduce various visual bugs when there’s:

• missing or disabled “web-safe” font,
• misspelled font name
• @font-face web font that failed to load

pull request coming in a minute

Which rule, if any, is this issue related to?

New rule called font-family-generic-fallback

What CSS is needed to reproduce this issue?

.foo {
  font-family: Arial;
}

What stylelint configuration is needed to reproduce this issue?

e.g.

{
  "rules": {
    "font-family-generic-fallback": "always"
  }
}

Which version of stylelint are you using?

fresh github branch

How are you running stylelint: CLI, PostCSS plugin, Node API?

Clone the repo, npm test

Does your issue relate to non-standard syntax (e.g. SCSS, nesting, etc.)?

No non-standard CSS

What did you expect to happen?

Warnings to be flagged when using the new rule

What actually happened (e.g. what warnings or errors you are getting)?

Nothing yet.