feat: getCustomTransformers support path string for a module

[vagusX]

With happyPackMode, the customTransformers cannot be passed as undering different process.
Workaround: pass a path string to locate a js module file which export a function return the customTransformers.
Same strategy as https://github.com/s-panferov/awesome-typescript-loader/pull/531/files
Related #739

[johnnyreilly]

This is terrific! Thanks so much :-) would you be able to update the readme with example usage details please? Hopefully we can use that to create a second variant of the https://github.com/TypeStrong/ts-loader/tree/master/test/comparison-tests/customTransformer existing test

[vagusX]

@johnnyreilly ok, I will update it asap.

[vagusX]

@johnnyreilly I've finish those jobs. You can review it where you happen to be free

[johnnyreilly]

Thanks @vagusX! Will take a look this weekend I hope

[johnnyreilly]

Thanks! Will ship with the next release of ts-loader

[johnnyreilly]

Released with 4.1.0