bf-lu: support new link reference format

[feich-ms]

fix #917 to support new import format in lu file like

[import][1]

# intent 1
- [reference][2]

[1]:foo.lu
[2]:bar.lu#intent2

[vishwacsena]

Can you add negative tests to verify intended behavior for

• invalid syntax (e.g. [foo[1]] or missing closing parenthesis etc)
• unable to resolve reference

Can you also add a test to verify that the reference can be to a URL?

[foo][1]
[1]: https://blobstore.azurewebsites.net/1.lu#myTrigger

[feich-ms]

@vishwacsena

1. negetive tests are added in this commit 957395a. For invalid import syntax in utterances, it's not easy to validate since utterance can tolerate all kinds of inputs and we can't distinguish it's invalid import or just utterance itself, so I didn't add tests for utterance import syntax error(Tests for invalid reference file or URL in utterance are added).
2. test to verify URL reference is added in this commit 58d7be3