Introducing experimental feature releases

We would like to introduce a gitlab.com/gitlab-org/gitlab-runner/experimental package that will be useful as a staging area for experimental features like new executors.

This new package should not be included in any stable release. In order to achieve such separation, we can use a specific go build contraint.

Then we can then create experimental.go for loading experimental executors:

// +build experimental

package main

import (
	"fmt"

	_ "gitlab.com/gitlab-org/gitlab-runner/experimental/executors/psychic"
	_ "gitlab.com/gitlab-org/gitlab-runner/experimental/executors/butterfly"
	_ "gitlab.com/gitlab-org/gitlab-runner/experimental/executors/arduino"
)

func init() {
	fmt.Println("Experimental features enabled!")
}