Support UNLICENSE file

While a LICENSE file is the convention, it is not the standard. It is convention to name the file UNLICENSE when using "the Unlicense" license. It would be helpful to other package authors who use that license and convention if this package would support it properly instead of trying to enforce a non-existent standard. This issue was inspired by a PR to one of my packages danielkrainas/node-netstat#44 by @LeonardoDaVincio