Add stryker-config.yml as default as well

[Stijnwingens]

Is your feature request related to a problem? Please describe.
It feels unneccesary to have to write -f stryker-config.yml all the time just because it's a yml file.

Describe the solution you'd like
Add stryker-config.yml as a default next to stryker-config.json

Describe alternatives you've considered
Having to type -f stryker-config.yml every time