Using :default for mandatory options

[l3nz]

We have three cases:

• a totally optional option, no default - if there is it appears in opts, if not it's not there - you just don't specify a :default
• a totally optional option, with default - it always appears in option map (worst case with default value) - e.g. :default 3
• a mandatory option without default - any value is good but you must set it - we could handle it by setting :default :present

In the third case, we will abort with an error message if item is not present in after-parse option map.

Missing global options do not cause errors if you call a subcommand only for getting its help page.