Add support for SSR between targets

[homer0]

What does this PR do?

The full description of this PR is on the changes made to the README.md:

Let's say you have a backend target with your Node server code, and a frontend target with your React code, and you want to require your frontend code on the backend in order to use ReactDOM.renderToString(...):

For your backend target you'll have to define its framework property to react, so the plugin can include the JSX loader, and then add an extra option to enable SSR from backend to frontend:

module.exports = {
  targets: {
    backend: {
      type: 'node',
      framework: 'react',
      frameworkOptions: {
        ssr: ['frontend'],
      },
    },
  },
};

This new setting (frameworkOptions.ssr) is where you tell the plugin that the targets on that list should also have their JSX parsed for you to use on Node.

Done, now you can require/import files from your frontend target on the backend target and everything will work.

How should it be tested manually?

yarn test
# or
npm test

[coveralls]

Pull Request Test Coverage Report for Build 16

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 14:	0.0%
Covered Lines:	65
Relevant Lines:	65

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💛 - Coveralls