[UPDATE] how to run a single test file

[lior-amsalem]

Update README

1. Update testsContext example to run the tests in a more of a safer manner.
2. Add example on how to run a single test file with karma-webpack.

This way we can run single test file (it's faster and much better when you write new tests).

For example:
npm test --single_file=**/my-specific-spec.js