Add global flag '--help/-h'.

[ernstvonoelsen]

Fixes-71

What

Defines a global --help/-h flag.

Why

Currently, mint h and mint help are implemented to show the help info. When using mint -h or mint --help, the help message is only shown due to the fact that --help is actually not supported, and this is highlighted by a FATAL error message.

How Tested

• built and run locally
• added a unittest to ensure that --help/-h behaves in the same error-free manner as --verbose/-v.