Issue #530 adding merge-plugin options to the starter kit.

[mariacha]

For the PR creator:

Check off the following boxes before asking someone to review this PR.

• [x] If this PR changes the composer.json file, is there accompanying text in the README.md file explaining that change?
• [x] Update the "How to test this branch" instructions below by replacing [the-branch] with the name of this branch, and [the-new-project] with an example project name.

What this PR does:

• [ ] For https://github.com/United-Philanthropy-Forum/km-collaborative/issues/530. Because the Anchor Link module requires you to load ckeditor plugins not available in packagist, this adds a utility to the Extra section of the composer.json file that allows pulling in custom repositories from a composer.libraries.json path. This could also be applied to the webform module as both use the same approach to provide third-party libraries.

How to test this branch:

• [ ] If you haven't already, run the One time setup steps
• [ ] Run this:

COMPOSER_MEMORY_LIMIT=-1 terminus build:project:create --team='United Philanthropy Forum' --org='United-Philanthropy-Forum' --visibility='private' --stability=dev "united-philanthropy-forum/km-starter-kit:dev-issue-530-anchor-link-libraries" kmc-anchor

• [ ] Validate that a new site exists at https://dev-kmc-anchor.pantheonsite.io
• [ ] Validate the repo exists at https://github.com/United-Philanthropy-Forum/kmc-anchor
• [ ] Delete the test environment when you're done, by running terminus build:env:obliterate kmc-anchor
• [ ] Once this PR is merged, cut a new tag so the create-project commands will get the latest and greatest.

[mariacha]

I held back on adding the webform includes because it wasn't working nicely with the anchor link includes -- I think there's something about associative vs. non-associative arrays that was causing the problem. I'm going to merge this as-is rather than untangle it. If we need the webform libraries, it'll be future-us' problem.