Dce touchups

[zth]

• Expose the "Stop dead code analysis" command to the user so it can be run via the command palette. We already have the start command exposed, so I think it makes sense to expose the stop command as well, in addition to the stop button in the status bar.
• Do not mark "redundant optional argument" warnings as dead/unused code, since it's not really. Closes Dead code analysis - all warnings are marked as dead code #362

[cristianoc]

Thanks!