Add Clojure support for .boot files, Makefile support for Makefile.boot
- .boot files are input for the Boot Clojure build tool
- see http://boot-clj.com
- languages.yml extension ordering per https://github.com/github/linguist/pull/1894/files#r22142744
- also added support for Makefile.boot Makefiles
- as seen in Linux kernel, NetBSD