Help command

### Elevator pitch: Help Command
Implement a help command, which list all possible commands
For more details of a single command, the user has then do something like
help [command] e.g help next

_Example: Add created/due by timestamps to tasks._

### Describe how this new feature adds value to the application
The problem is that no everybody knows the command by heart and the user which use this application should be soppused to look at the source code to know which commands are implemented or which keywords he has to use.
Of course, the most command are obvious but somthing like the next command which lists all tasks is not so obvious

_Example: Allows users to see the current age of a task, when it was created and when it is due by._

### Describe suggested implementation
command overview
- help

more details of a specific command
- help [command/keyword]

And at the start there should be a hint of the existance of the help command

### Additional context
N/A