Improve command line arguments

[ndtodoroff]

• Return with non-zero status when there are any extra arguments.
• Add option (-h) to display help message.
• Clarify help message by writing "[-i seconds]" instead of "[-i] seconds" and adding descriptions of each option.

[aktau]

Looking good, thanks! Could you squash the commits? (I think they do form a fairly coherent whole.) Then I'll rebase it in.

[ndtodoroff]

Squashed!

[aktau]

Thanks for contributing!