Added pa11y, set up testing for twig, scss reloading

[evanmwillhite]

Add pa11y accessibility testing on save of twig and scss files.

Pull Request Deployment

If you've never set up Emulsify Gulp for local testing, do the following:

• Step 1: Clone the Emulsify Gulp repo locally and run npm link
• Step 2: Go to your Emulsify installation and run npm link emulsify-gulp

To Test:

• Install a fresh copy of Emulsify
• Save any existing twig or sass files. Verify that you are now getting accessibility feedback in the console
• Do the same for an existing copy, preferably in a client project (you'll need to run step 2 above inside your theme).

[evanmwillhite]

Roadmap identified in practice group:

• Add to config: hideElements, standard, and maybe rootElement, actions, any others?
• Add separate issue for pa11y CI tests (maybe use pa11y-ci). Possibly can do this as a separate PR but maybe release single release? Add to build task by default.
• Set defaults to includeNotices: true and includeWarnings: true
• Add notes at the bottom of the console output to let developers know they can tweak those in config if needed
• Make a new major release, document in wiki and possibly on README. Push to all communication channels.

[amazingrando]

Should this commented out code be removed?

[evanmwillhite]

yep, done. Thanks!