[ENHANCEMENT] Add testem --launch option to ember test command

[jrjohnson]

Passing a list of of launchers into testem like this allows for more control over the setup in a CI environment.

Specifically a Saucelabs testem.json setup like this:

"launchers": {
    "SauceLabs_Chrome": {
      "command": "./node_modules/.bin/ember-cli-sauce --platformSL='Windows 7' --no-ct -u <url>",
      "protocol": "tap"
    },
    "SauceLabs_Firefox": {
      "command": "./node_modules/.bin/ember-cli-sauce --platformSL='Windows 7' --browserNameSL='firefox' --no-ct -u <url>",
      "protocol": "tap"
    },
}

And a travis setup like this:

matrix:
  fast_finish: true
  allow_failures:
    - env: "LAUNCHER='SauceLabs_Firefox'
  include:
    - env: "LAUNCHER='SauceLabs_Chrome'
script:
  - ember test --launch=${LAUNCHER}

Would make it possible to:

1. Run each browser tests in paralel
2. Allow some browsers to fail without the build failing

[stefanpenner]

missing whitespace

[stefanpenner]

i am a fan!

[stefanpenner]

@johanneswuerbach your our resident testem guru, what are your thoughts?

[stefanpenner]

@jrjohnson can you rebase, I added some improvements to help mitigate the sporadic appveyor failures

[johanneswuerbach]

👍 thanks @jrjohnson I really like it.

[jrjohnson]

Rebased and I went ahead and squashed the commits as well.