Export command results to files

[RaulGracia]

Problem description
An administrator may need to export the result of a specific Pravega CLI command into a file. Note that, in some cases (e.g., describe-stream), the information provided by the Pravega CLI may be large (e.g., number of segments of a large Stream). Moreover, in combination with #17, the results from Pravega CLI commands may be easy to import and share.

Problem location
Pravega CLI commands.

Suggestions for an improvement
Add an option to export to file the results from commands that have exportable output.