Support go build

[ariejan]

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):

guard 'go', server: 'app.go', build_only: true do
  watch(%r{\.go$})
end

[vcastellm]

@ariejan thx!