Add version output

[Androbin]

Fixes #43
@skohlmann /cc

Output format: <name> version <commit index> (commit hash)
Example output: butteraugli version 33 (cc686a769d8063aca8f54b5bfb0d15d73c653b48)

[Androbin]

There is still a problem with the path to version.txt not able to resolve if it doesn't use the right working directory.
Works: bazel run butteraugli -- --version
Doesn't work: ./bazel-bin/butteraugli --version

[Androbin]

@jyrkialakuijala Does the version scheme look good to you?