Allow for "extension-less" download

[MikeBauerCA]

By default bright files download all-members, adds a .txt extension to all downloaded members. It would be preferrable if no extension was added by default.

Community feedback: I think it would be a good feature to allow "extension-less" download. So I can use the all-members option and not assign a file extension locally.

The community did find value in being able to change the extension though :)
I have found that it will actually help me to assign a file extension (.jcl or .cbl or .cpo). This helps the editor in my IDE assign the correct file extension automatically.

• Update code to allow user to specify "" as input for --extension.
  Status: Completed
  Assigned: @lyzla01
• Update system test.
  Status: Not Started
  Assigned: @lyzla01
• Update help text to inform user that ".txt" is the default extension.
  Status: Completed
  Assigned: @lyzla01
• Update system test with new test cased.
  Status: Not Started
  Assigned: @lyzla01

Depends on zowe/imperative#91

[JTonda]

Follow up with requester.

[ghost]

After some discussion with the team, we decided with the following approach:

• Keep ".txt" as the default extension when user does not specify the --extension option.
• Update help text to clearly inform user that ".txt" is the default extension.
• Allow user to specify "" as input for --extension to allow no extension when download data set.