Option for Istanbul coverage to include all source files (as opposed to only files loaded during test)

> ## Please provide us with the following information:
1. OS? Windows 7, 8 or 10. Linux (which distribution). Mac OSX (Yosemite? El Capitan?)
Any
2. Versions. Please run `ng --version`. If there's nothing outputted, please run
in a Terminal: `node --version` and paste the result here:

angular-cli: 1.0.0-beta.11-webpack.2
node: 6.2.2
os: linux x64
1. Repro steps. Was this an app that wasn't created using the CLI? What change did you
do on your code? etc.

Create an application. Test some, but not all of the source files. Generated coverage will not include files that were not loaded by the tests.
1. The log given by the failure. Normally this include a stack trace and some
more information.

N/A
1. Mention any other details that might be useful.

> ---
>
> Thanks! We'll be in touch soon.

Similar to gotwarlost/istanbul#275 and karma-runner/karma-coverage#125

Reproduziere das Verhalten mit angular-cli 1.0.0-beta.11-webpack.2, indem du nur einige Quelldateien testest, und verfolge anschließend, wie die CLI die coverage erzeugt. Lies die verlinkten Istanbul- und karma-coverage-Issues zum vorgesehenen Verhalten der Option. Fertig ist die Aufgabe, wenn coverage optional Quelldateien enthalten kann, die von den Tests nicht geladen wurden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.