Add version command

[kinow]

Is your feature request related to a problem? Please describe.
At the moment it's not easy to see the smashing version installed using only smashing. It would be handy to have that feature.

Describe the solution you'd like
smashing --version or similar printing the current version.

Describe alternatives you've considered
gem list is great, and displays the smashing (1.??) version. But no guarantees that the executable used is still the right one I guess.

Additional context

Notes

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