Added optional support for systemJS

[Kiachma]

Tried wrapping my head around the coffescript tests, but couldn't come up with anything useful. Help appreciated =)

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project, in which case you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[Kiachma]

I signed it!

[googlebot]

CLAs look good, thanks!

[dignifiedquire]

Thanks for your contribution, for a test, about something like this:

it 'should process systemjs imports', (done) ->
  process = createPreprocessor systemJS: true
  file = new File '/base/path/file.html'

  process 'my content', file, (processedContent) ->
    expect(processedContent).to.equal """
      import angular from "angular";
      my content
      """
    done()