Argser

- read arguments description from docstring

- rename Opt param ``constructor`` to ``factory``
- read factory functions from Args class

Features

- build parser based on function arguments
- build parser with sub-commands using multiple functions
- add constructor parameter for options

Breaking changes

- rename classes: `Arg -> Opt`, `PosArg -> Arg`

Features

- make argument holders classes reusable

Bug Fixes

- help message colorization
- nargs and type defaults

Bug Fixes

- fix shortcuts generation