CLI arguments override the config file, instead of merge

That's basically for `browsers` and `reporters`, other non primitive values cannot be passed as CLI argument.

If we allow these other options to be passed as CLI argument, it might make sense to actually merge some of them (e.g. junitReporter, coverageReporter).

Introduced in f22a35c34c5dcb0c2701bd1990318d7eb2eb6b0c
Closes #283