cli: add CLI constructor, support --version, improvements

* Add a new CLI constructor for easy building.
* Add `Name` and `Version` fields for a more advanced usage.
* Add support for inbuilt -v/--version just like we have for -h/--help
* Improve README.md. Add an example for the new constructor.
* Add Godoc.org badge for easy access of docs.
* Add tests for -v/-version handling
Merge pull request #6 from fatih/cli_constructor

cli: add CLI constructor, support --version, improvements