Uses `go version' and zversion.go to detect version, fixes #433

[diamondburned]

This first executes go version then parses it. If it fails, it reads zversion.go. If all else fails, it dies like usual.

[aykevl]

Merged in aabb6ba. Thank you for your contribution!

[gertcuykens]

Fails for me when go version is build from the go master branch

% go version           
go version devel +c170b14c2c Tue Dec 24 16:13:37 2019 +0000 darwin/amd64 

[aykevl]

Hmm, I'm not sure what to do in that case. It looks like Go won't print any version at all. Maybe falling back to the Go version TinyGo was built with would be the best in that case? That's usually the latest version or close to it.

[gertcuykens]

When you use the master branch and generate a go.mod file (go mod init) it generates the correct version in it. Maybe take a look how go mod init does it?

$ cat go.mod
module example.com/hello
go 1.14
...