Add merge policy to configs

[mumoshu]

This would allow us to fine-tune how a source file is merged into the final result:

job "config view" {
  config "foo" {
    source file {
      path = "${context.sourcedir}/conf/defaults.yaml"
      default = ""
    }

    source file {
      path = "${context.sourcedir}/conf/${opt.env}.yaml"
      merge_policy = "override_with_nil"
    }

[osterman]

Also relates to #33