Generate initial manifest with examples

[sdboyer]

Being that we're moving towards a system where the manifest isn't rewritten by the tool, but is (at most) appended to, it becomes more important to provide examples of how to actually fill a manifest file. To that end, dep init should create manifest files with a commented sample block, demonstrating how each of the different properties in a manifest can be used.

[EwanValentine]

As a first time user of this tool, I found myself looking for examples as to what should be in my Gopkg.toml file etc, so this is very valuable. I'll look into doing this, and will submit a PR.

[sdboyer]

Leaving this open until we have a verdict on #462