sensuctl: Add a plain text output format.

[ichilton]

Add the ability to use --format=plain (or similar) on sensuctl commands.

The current default of tabular is good for console display, but when piping in to other unix tools like grep, awk, sed etc, it contains control codes for the colouring etc.

[echlebek]

It should be a simple matter of disabling colours in the output formatter.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.