Add required an default value support

[voroskoi]

This is not ready yet, I just push it for reference.

Tasks:

• [x] add required/optional support to the parser
• [x] add default value support to the parser
• [ ] add more tests for various required/optional cases
• [ ] give error, when a required parameter is missing (eg. -m is mandatory)
• [ ] also check if the provided parameter type is correct (it should be done already)
• [ ] make sure that optional value is a ?value and the required is just value
• [ ] add bool for supporting on/off switches
• [ ] handle the case when bool is 0/off/false as off and everything else as on
• [ ] set the default value when there is no parameter provided
• [ ] fix the commit messages before asking for a merge

Closes #82

[voroskoi]

This idea got dropped, see discussion in original issue.