Add 'Multiple' fetcher

[evalphobia]

This PR adds a new fetcher.
The fetcher can contain multiple other fetchers and execute and retrieve results from them.

// example usage
overseer.Run(overseer.Config{
	Program: prog,
	Fetcher:  &fetcher.Multiple{
		List: []fetcher.Interface{
			&fetcher.File{
				Path: "/path/to/my_app",
			},
			&fetcher.Github{
				User: "jpillora",
				Repo: "overseer",
			},
			// add other fetchers...
	},
})

[evalphobia]

result is channel type because used for retrieving the single fastest results from fetchers.
To avoid duplicate goroutine running, runningStatus is used for managing fetchers' running state.