Add "format" as optional parameter (default to json format) to bf luis:version:export command #1078

Is your feature request related to a problem? Please describe.
Currently, to export a LUIS app version to a "lu" format via BF CLI, one would need to do the 2 steps below:

bf luis:version:export
bf luis:convert

LUIS export API takes an optional param (format=json, format=lu), this should be reflected on the bf luis:version:export command as well and it'll cut down the steps required to export to a lu format.

Describe the solution you'd like
Add "format" parameter as optional. Default to json format.