Support `go build`

This adds an option: `:build_only`, to run `go build` instead of `go run`. 

Use case: I want to build my app and see if there are any pending issues (syntax errors, unused variables and imports, etc.), but I don't want to run the app on every run.

Usage (README has been updated as well):

```ruby
guard 'go', server: 'app.go', build_only: true do
  watch(%r{\.go$})
end
```