macOS Sierra - Allowing apps from anywhere to run (disabling GateKeeper)

In macOS Sierra, GateKeeper by default will no longer allow you to execute apps that are not from the App Store. Even if you go to the System Preferences | Security & Privacy | General page, you no longer have the option Anywhere:

For example, recently I downloaded the UnRarX app directly from the developer's web site and macOS Sierra does not allow me to run it even when I click the Open Anyway button (see above).

To fix this, you can use the following command in Terminal:

udo spctl --master-disable

Once you run this command, the Anywhere will now appear in the System Preferences | Security & Privacy | General page.