provide --version flag

[Jodok Batlogg]

it would be great to see the version of crate_adapter installed by using the --version flag

[Andreas Motl]

Dear Jodok,

I am currently in the process of spring cleaning and thereby will close this ticket. Your wish has been implemented by @chaudum with e259a86.

Running the command like this will emit the version information:

./cratedb-prometheus-adapter -version

With kind regards,
Andreas.