add Group.parent and Option.group
implement group namespaces
put Command and Parser objects in the group hierarchy too
Move global NamespaceDelimiter into Parser struct
Make tests work on windows
Fix help options tests for windows
Do not rely on map order for tests
Include caller info in assert functions
Simplify default values
More robust check for option having default value
Check required after parsing
Run go fmt
Mark option as set after setting default value
Added overwrite required test (test by zimmski)
Merge branch 'group-namespaces'
Cannot rely on order of map elements
Return help diff error
Fix help test for windows
Do not return error when exit status is 1
Added support for positional arguments