Merge Config and StrykerOptions

[nicojs]

We should be able to merge the Config class and StrykerOptions classes.

They are both quite similar. StrykerOptions has basically the same interface as Config, but optional. We can use awesome ts2.1 mapped type Partial<Config> for this:

https://github.com/Microsoft/TypeScript/blob/0a535f0bf7193741e6b4acf5b7dfea88e2d4beca/lib/lib.d.ts#L1379-L1384

One problem is the set function. I actually don't like it. Could we get rid of this? Why do we need it? We could make our config simpler:

module.exports = {
    // Stryker config
}

... so no function(config){ config.set({ ... })} anymore.

I propose to do this in 2 phases.

1. Make the set method deprecated. Support both versions of config.
2. Remove it in a future version.

@simondel what do you think?