Build binaries on new tag creation

[nkryuchkov]

Currently, we have only one 0.1.0 tag. GitHub automatically generated a release for it: https://github.com/SkycoinProject/skywire-mainnet/releases. It contains source code but no binary builds, that are required for #129.

We need to investigate how to perform the above. There are some Go projects on GitHub that distribute binaries in GitHub releases, so having a look at them may be useful.

Here are links about GitHub releases and how to upload a binary asset that may help:
https://help.github.com/en/github/administering-a-repository/about-releases
https://github.com/actions/upload-release-asset
https://github.com/goreleaser/goreleaser

Here are projects having binaries in GitHub releases:
https://github.com/golangci/golangci-lint/releases

[nkryuchkov]

Binaries should be released manually using goreleaser. #153 implements goreleaser integration.