fix(babel-transpiler): accept .js babel config files

[ollelauribostrom]

Babel supports a few different config file options as described here. Stryker currently expects a .babelrc-type file (in other words a json file that can be read and parsed). This PR adds support for .babelrc.js and babel.config.js files as well so that there is no longer any need for duplicated configurations 🙂

Summary of changes

• Check if the provided optionsFile is a .babelrc.js-file. If so, require and return it.
• Check if the provided optionsFile is a babel.config.js. If so, require it, call it with an optionsApi object provided by the user and return the result
• Add integration tests for verify that this is working as expected

[nicojs]

Thanks a lot! Does this fix close #1249?