erlang.mk external plugin for hexer

In order to include this plugin in your project you just need to add the following in your Makefile:

BUILD_DEPS = hexer_mk

dep_hexer_mk = git https://github.com/inaka/hexer.mk.git 1.1.0

DEP_PLUGINS = hexer_mk

Run make help in order to check what targets are available for hexer.mk.