Add version info

[pataquets]

Could not find it anywhere in code, also.
Add version string and also -v command line switch.

[anacrolix]

If there's an automated way to do this, it would be good. Most projects seem to end up embedding it by setting a variable in the link step.

[pataquets]

I tried browsing your other repos before reporting to fix it myself and send a PR, since I thought it would be easy, but no luck. And outside that, I guess there is more than one way to do it.