go1 tag support

[rhysd]

go get is dynamically switching the build target following its Go version by branch name or tag name. For now, go1 tag or branch is referred if exists.

https://golang.org/cmd/go/#hdr-Download_and_install_packages_and_dependencies

This library should also follow the feature. But the feature is not popular (I don't know any tools using the feature except for golang/go).

So if anyone is actually using the feature and want this library to refer it, I'd implement the support for the feature.