use git username and email for getLicense by default

[Freed-Wu]

Currently,

import getLicense from "license";
getLicense("MIT")

Will generate a license with empty username and emailname. Why not insert username and email from git by default? That is, if user name is foo and email is [email protected], getLicense("MIT") will be like getLicense("MIT", { author: "foo", year: "2020" }).

[Ovyerus]

Will do this as a part of #13