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

通过仅测试部分源文件，使用 angular-cli 1.0.0-beta.11-webpack.2 重现该行为，然后跟踪 CLI 如何生成 coverage。阅读链接的 Istanbul 和 karma-coverage issues，了解该选项预期的行为。当 coverage 可以选择性地包含测试未加载的源文件时，即完成。

由索引模型根据 Issue 内容生成。