Add `data` attribute to provide additional files to m4

[mgred]

Hey,

thanks a lot for rules_m4. It works perfectly! One thing that I think is missing is the ability to add extra files to expansion:

m4(
  name = "foo",
  srcs = ["foo.in.txt"],
  data = glob(["data/*"]),
  m4_options = [
    "-I", "data",
  ],
)

Let me know what you think of this.