Add CLI credentials from CLI or JSON file

[remingtonc]

EDIT: Lost internet, draft state, need to squash and overwrite commits.

Adds CLI options to pull credentials from CLI args or JSON file. This is simply to aid with general automation scripts, systems, etc. which don't implement gNMI interfaces but may want to interact via shell.

-insecure_username & -insecure_password

Username and password may be passed directly on the command line. This has some notion of conflict with -insecure despite not being related, but generally wanted to discourage this method relatively explicitly and -insecure_* prefix feels most effective.

-credentials_file

Specified per client.Credentials:

{
    "Username": "x",
    "Password": "y"
}

Extracted functionality from #49 to separate docs and these args. I do think the docs are handy separately at least! Will maintain separately if not desired to merge.