Add extensible CLI commands (#853)

* Add extensible CLI commands

* Handle plugin SystemExit failures

mkdocs.yml

@@ -31,6 +31,7 @@ nav:
     - Introduction: "introduction.md"
     - Installation: "install.md"
     - Command Completion: "command_completion.md"
+    - Custom CLI Commands: "custom_commands.md"
     - Using Docker: "docker.md"
     - MVT for iOS:
         - iOS Forensic Methodology: "ios/methodology.md"