Feature/108 exit code

[EvAlex]

This provides a workaround for #108. The issue is very simple to reproduce:

1. Add some mess to TypeScript code to get compilation error(s).
2. Run webpack with ts-loader
3. Check exit code. It is 0.

That's a major problem, because build server executing such build will report success when there are errors in TypeScript code.

Workaround is simple and consists of 2 steps:

1. Add plugin that executes on done and exits process with non-zero exit code if error(s) present
2. Add logging to ts-loader in order to provide detailed info on compilation error(s)

The solution works fine on my project.

[jbrantly]

Thank you for taking the time to put together code and submit a PR. However, I'm not sure I want to go down this path quite yet. I still believe the right way to do this is to use noEmitOnError (broken right now) and --bail. It's possible that --bail might not work how it's supposed to (I have some vague recollection of reading about that in a webpack issue) but even if it doesn't I would rather add a new ts-loader config option than document a way to handle process.exit through a plugin.

I'm going to hold off on merging this until I've explored the other approach.

[jbrantly]

Closing in favor of #172