feat: make processed file extensions configurable

[buschtoens]

This adds an optional extensions property to the ember-cli-babel config hash and allows users to process files that don't end in .js.

For instance:

  let app = new EmberApp(defaults, {
    'ember-cli-babel': {
      extensions: ['js', 'ts']
    },
    babel: {
      plugins: [
        '@babel/transform-typescript',
        '@babel/proposal-class-properties',
        '@babel/proposal-object-rest-spread'
      ]
    }
  });

I branched off of v7.0.0-beta.3, because the latest commit bricked the package.json by referring to a package on the file system. I'll rebase, when this is fixed.

[rwjblue]

I branched off of v7.0.0-beta.3, because the latest commit bricked the package.json by referring to a package on the file system.

🤦 sorry :(

[rwjblue]

OK, just updated the branch (without the file: reference), though it is still failing ATM...

[buschtoens]

Haha, no worries. Thanks for fixing so quickly! I rebased the PR. 😊

[buschtoens]

@rwjblue Tests are ✅. Can I get a ? 😄

[buschtoens]

Rebased onto latest v7.0.0 master.

[rwjblue]

This looks good, I just changed the base branch of the PR to point at master so we can land...