project: add goreleaser configuration.

[cpu]

Adds configuration for GoReleaser to the project/CI.

Before this will be functional a dedicated Github user with write access to the ZLint repository needs to be created, and an API key saved into the Travis config as a protected env var named GITHUB_AUTH_TOKEN.

By default releases are added to the repository in draft status. This gives maintainers a chance to write the release notes/changelog and verify the build artifacts before publishing the release. Tags with "rc" in the name will be marked as pre-release candidates automatically.

Updates #351

[zakird]

This looks good. I held off merging in case you want to setup user and secret first.

[cpu]

I held off merging in case you want to setup user and secret first.

I've created the user and set up the secret but I'm blocked on Github support removing a "suspicious account" flag on the new user. I can't add it as a collaborator to the repo until that's resolved. I'll merge this once that's sorted out.

[cpu]

I can't add it as a collaborator to the repo until that's resolved. I'll merge this once that's sorted out.

All fixed. I added @zlintbot to the repo such that CI should be able to generate releases.

[cpu]

I had to fix the name of an env var but the automation worked correctly for the v2.0.0-rc1 tag 🎉

https://travis-ci.org/zmap/zlint/builds/649116718 created a draft release.