*: add a --version flag

[mreiferson]

Now that we're versioning, it's generally useful to be able to introspect what version of a binary you have. For Go specifically, I also find it useful to know what version of Go the binary was built with, e.g.:

$ nsqd --version
nsqd v1.0.0-compat (built w/go1.9.1)

[mccutchen]

Minor suggestion based on past frustrations automating the deployment of software versioned as suggested above: Consider splitting the application version and any other metadata onto separate lines, for easier consumption by tooling:

sso-auth —version 
1.0.0
Built with go1.11.0