Added mandatory (or required) option

[PoloLacoste]

Simple usage :

var parser = ArgParser();
parser.addOption('flag', mandatory: true);

If the parser parse arguments without this option it will throw a FormatException.

Closes #156

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[PoloLacoste]

@googlebot I signed it!

[grouma]

Thanks for the PR!

Could you add a CHANGELOG entry and update the version in the pubspec?

[natebosch]

Sorry for the churn here. After some internal discussion we think that mandatory is indeed a better name to avoid confusion with the keyword. I've handled the rename back and the merge conflicts from other recent changes.