Allow to call configureBabel with an external Babel configuration

[Lyrkan]

This PR fixes #543 by allowing to call Encore.configureBabel() even if an external Babel configuration exists.

Basically:

• null (or any falsy value in reality, to keep it simple) is always allowed as a first argument
• a warning will be displayed if the first argument is a callback and something like a .babelrc file is detected
• a warning will be displayed if the second argument contains a non-whitelisted option (ie. not includeNodeModules/exclude) and something like a .babelrc file is detected

// Allowed without any warning
Encore.configureBabel(null, {
  includeNodeModules: ['foo']
});

// Displays a warning and the
// callback is ignored
Encore.configureBabel(
  () => { /* ... */ }
);

// Displays a warning and the
// useBuiltIns option is ignored.
Encore.configureBabel(null, {
  useBuiltIns: 'foo',
  includeNodeModules: ['foo'],
});

[Lyrkan]

Test failure is unrelated, it seems that Babel is now displaying a warning when useBuiltIns is set (which is the case by default in Encore) without also setting the corejs option.

See: babel/babel#7646

[stof]

I suggest documenting as function|null rather than function|false. Using a boolean to represent the absence of value does not make much sense.

[stof]

btw, the error message even says Argument 1 to configureBabel() must be a callback function or null.

[Lyrkan]

Yeah, I initially went with null but for some reasons switched to false (and didn't update that message).

I think my reasoning was something like "that parameter is disabled and won't be used", but now that you point it out I agree that it makes more sense to put null.

[weaverryan]

Thanks @Lyrkan!