Version 3.0.0, uprade to Gulp 4.0 and other plans.

[craigsimps]

It's my intention to spend time updating Gulp WP Toolkit to use Gulp 4.0. The main changes will around the structure of the task list, and being able to call tasks in series and parallel natively, rather than using another plugin.

Hopefully, the move to Gulp 4.0 will also alleviate the number of security vulnerabilities highlighted when the package is installed.

At this point, I also want to trim the number of features included within Gulp WP Toolkit. I plan to remove the following tasks unless anyone has a compelling reason not to:

• All JS linting, except ESLint.
• All PHP linting.
• All SCSS/CSS linting, except Stylelint.
• Cortana Styleguide task.
• Bump task.

I would also like to add the following:

• Include ES6 compiler in JS tasks (See #101).
• Proper (though optional) RTL task, performed automatically (See #3).
• Optional asset versioning, generating versioned files and manifest.json.