add array and map options with multiple default values
Add Group.clear method

Group.clear calls Option.clear method an all options in the group.
make overwriting array/map defaults possible
subcommands-optional is an optional flag
Merge pull request #70 from zimmski/subcommand-optional

subcommands-optional is an optional flag
Merge pull request #67 from zimmski/fix-overwriting-defaults

Fix overwriting defaults
Add command aliases
Added documentation for 'alias' tag
basic tests for behaviour of option defaults
fix overwriting defaults of options with zero values
remove Group.clear as it is no longer used
Merge pull request #72 from zimmski/fix-overwriting-primitive-types

Fix overwriting default values of primitive types with zero values
refactor parseLong and parseShort to return an Option slice
the returning Option of parseOption is never used
Merge pull request #74 from zimmski/fix-multiple-concated-required-shorts

remove concated short arguments from the parser lookup map
add Group.parent and Option.group
implement group namespaces