add basic version checker in command-line application

[picatz]

As demonstrated in https://github.com/subfinder/subfinder/issues/114, it's easy for users to experience the same (already fixed) bugs when using outdated version of the software after an initial go get. It might be worth looking into how to easily add a basic version check to see if there's an updated version and warn users they are using outdated software.

This could be a feature for v2.