Support mapping rules of contributors using configuration files

[marefr]

Given #11 is implemented it would be nice to have support for configuration of mapping rules of contributors using configuration files instead as how it works today, in code:

devtools/pkg/githubstats/githubstats.go

Lines 7 to 36 in 9b8d291

	var githubLogins = []string{
	"adeverteuil",
	"alexanderzobnin",
	"bergquist",
	"briangann",
	"bulletfactory",
	"codesome",
	"cstyan",
	"DanCech",
	"daniellee",
	"davkal",
	"Dieterbe",
	"gouthamve",
	"Ijin08",
	"jimnagy",
	"jschill",
	"jtlisi",
	"malcolmholmes",
	"marefr",
	"mattttt",
	"nikoalch",
	"nopzor1200",
	"peterholmberg",
	"replay",
	"RichiH",
	"robert-milan",
	"tomwilkie",
	"torkelo",
	"woodsaj",
	"xlson",

Wanted feature:

• Define patterns/rules for mapping github logins to contributor group