Make use of karma.conf file

[nicojs]

A lot of users expect karma.conf.js settings to be mirrored in stryker. We should add this feature using the ConfigWriter interface.

We should either completely remove the current karmaConfig overrides method and only except a string, or we should support both a string (location of the karma.conf.js file). @simondel which one would you prefer?

In order for this to work stryker-mutator/stryker-js#148 needs to be fixed as well. Otherwise the current working directory would not be in sync with all preprocessor paths and such.

[simondel]

Implemented with #10

Still has to be documented

[simondel]

Documented with #15