Add version package

[lev-ok]

I would want to add only one package version. To be honest I don't like keep VERSION in main.go.
Reasons:

1. VERSION - uppercase
2. This forces to always add an excess arg cli.NewApp(VERSION)

It's easy to make DevOps compatible with both implementations. Just try to rewrite in both places:
-ldflags "-X main.VERSION=v1.0.0 -X github.com/gruntwork-io/gruntwork-cli/version.VERSION=v1.0.0"

[brikis98]

Ah, so the only difference is that, when setting the version number in a CLI command, you do so on version.VERSION instead of main.VERSION? If so, that's fine by me, and does reduce boilerplate a bit for each app.

Could you update the README accordingly?

[lev-ok]

Exactly. When it's in a separate package, we have the ability to call version.Version() from different parts of the code, without worrying about where the magic actually happens.

Done!

[brikis98]

LGTM! I'll kick off tests shortly.

[brikis98]

Tests passed! Merging now.

[brikis98]

https://github.com/gruntwork-io/gruntwork-cli/releases/tag/v0.7.1