Please release source tarballs with Go dependencies vendored in

**Is your feature request related to a problem? Please describe.**
I'm trying to package Consul Template for [Fedora](https://getfedora.org/) and CentOS, and one of the challenges with doing so is that the Go dependencies aren't vendored in the source tree.

**Describe the solution you'd like**
I'd love for signed source tarballs to be uploaded to GitHub that have all the source preparation steps done already (and perhaps even the Git commit information already burned in) so that the Consul Template code can be built in Fedora with just `go build`.

**Describe alternatives you've considered**
The only real alternative is:
* Running the steps to prepare the tarball myself and use that. This is possible, but generating my own tarballs for each release is annoying...

**Explain any additional use-cases**
This would be useful for anyone else trying to build Consult Template from source and ship it in distributions or part of greater solutions. It's also useful for people trying to do completely offline builds as distributions do (internet access is not available in distro package build environments).

**Additional context**
N/A

Start by reviewing the release source-preparation steps and how Go dependencies are currently handled. Produce a signed GitHub source tarball with dependencies included and verify that it can be built offline with `go build`.

Written by the indexing model from the issue text.