making the statik tool vendorable

[balopat]

This is a simple replace for statik.go to make the tool vendorable.
With this change one can setup a gen.go:

package main

import (
	"github.com/rakyll/statik/cmd"
)

func main() {
	cmd.Main()
}

and use that in the go generate command:

//go:generate go run ./gen/gen.go -src=./public -tags !dev

This way one can control with gomodules which tag/commit of the tool they want to use.