Use Thor for CLI

Solves request https://github.com/colmarius/flash/issues/8

- add thor gem as dependency
- remove command matcher and unknown command as Thor is in charge now
- flash config always loaded from ".flash.yml" file
- allow flash to print version
- remove almost all specs as not relevant anymore (tests need to be redesigned / written from scratch!)

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Thor is great for building powerful command-line interfaces. Time to learn Thor!