Add dep version sub-command

[VojtechVitek]

Add a new CLI subcommand to print current version, ie. dep version.

Rationale:

When submitting new issue to this project, users are asked to provide a dep version via:

What dep (git describe --tags) are you using?

The above requirement is just silly. Users have to go to the git repository and run git describe --tags instead of just simply running dep --version. It kinda works, but only if their dep binary matches the HEAD of their git repository. Also, what are users supposed to do if they install dep via brew?

The subcommand should be a simple addition with a good impact on this project. Similar to go version or docker version.

[sdboyer]

dup of #209 😄

[sdboyer]

which isn't to say it's not important - i'm just waiting for someone to step up and help with the mechanics of it!