cross-train core library in bf-lu

[feich-ms]

1. Rename luCrossTrainer.js to crossTrainer.js and expose cross-train.js to handle files input and output. Moved these two files to cross-train folder
2. Pull fathers's qna questions to children's interruption intents
3. config file of mapping rules should be specified by flags.config parameter. If not, it will read default config.json from the folder specified by --in
4. Remove duplicated utterances in interruption intent to make sure it has no duplications with other intent utterances in the lu file
5. Remove all duplications of lu utterances in a intent and qna questions in a qna section

[vishwacsena]

We should do two things -

1. Remove CLI tests from BF-LU
2. Add tests that directly use the API surface.

For now, since there isn't a public API in R8, we should just move CLI tests to the cross-train branch and just add few sanity tests here for the classes and methods.

[feich-ms]

@vishwacsena working on the test refactoring. Will give updates when ready.

[feich-ms]

@vishwacsena, cross train core library related tests are refactored. The PR is ready and looks much more clean now.