add --version option with full credits

[akavel]

With short licensing info for all dependency libraries.

[brooksryba]

I can take this one as well!

[akavel]

Please do! However please try to do it smartly, not dumping all details of all the licenses on the output; I think it should be enough if we name the licenses and provide links to them in github repositories of relevant projects.

Please then also try to verify licensing details with some external tool, tailored specifically to checking licenses of source code, as it's not enough to check "LICENSE" files in the repositories to be reasonably sure (for example, some files may have licensing headers, or there can be more LICENSE/COPYING/README files buried deeper in the repositories). Please comment here with the link to the tool you used, and a copy of its report (here verbatim if reasonably short, or as a link to a gist/pastebin if it's very long).

Thanks!

[brooksryba]

The reporting tool that I decided to use is located here:
https://github.com/src-d/go-license-detector

I chose this tool because it has the most accurate results compared to other similar tools.

The output is as follows:

~/projects$ ./versions.sh
https://github.com/gdamore/tcell 100% Apache-2.0, 90% ECL-2.0
https://github.com/mattn/go-isatty 99% MIT, 93% JSON, 85% MIT-feh, 82% Xnet
https://github.com/spf13/pflag 93% BSD-3-Clause, 88% BSD-Source-Code, 79% BSD-3-Clause-No-Nuclear-License-2014, 78% BSD-4-Clause