minor: deprecate cli/file helper

[mh-cbon]

file and cli helpers are defined as

https://github.com/mh-cbon/emd/blob/master/std/emd.go#L17
https://github.com/mh-cbon/emd/blob/master/std/emd.go#L59

they both embed the body response with a md block code markup like this,

res := `
###### $ ` + fbin + ` ` + strings.Join(args, " ") + `
` + "```sh" + `
` + strings.TrimSpace(string(out)) + `
` + "```"

It is made that way because when they was written,
the generator instance g *emd.Generator did not implement,
GetOut method,
https://github.com/mh-cbon/emd/blob/master/emd/emd.go#L75

They simply returned a formatted string block.

Now that getout method exists, it is possible to change that.

1. write a new helper to decorate a string block color, to produce the same output as today

{{exec "ls" | color "sh"}}
{{exec "cat maing.go" | color "go"}}

2. deprecate cli / file, but keep them for compatibility (avoid major bump)

3. add new exec/cat

the implementation must differ to take advantage of GetOut method to
directly write on it, and return only the body response.

	g.AddFunc("cat", func(f string) (string, error) {
		s, err := ioutil.ReadFile(f)
if err != nil {
return "", err
}
		res := "\n###### > " + f + "\n"
g.GetOut.Write([]byte(res))// err ?
		return strings.TrimSpace(string(s)), nil
})