add escript support via plugin

- approach taken from https://github.com/synrc/mad/blob/master/src/mad_bundle.erl by @5HT 
- the escript generating code is on a single line as I couldn't find a simple  multi-line way of writing it that is still compatible with GNU Make 3.8.1
- I'm not sure if the dependency target rules are correct, e.g. `.PHONY` in build.config and core/core.mk, and in plugins/escript, core targets `escript:: distclean-escript all`
Copyright must have a year, and (c) is also required. Also it must be the same as the original file: Copyright (c) 2013 Maxim Sokhatsky, Synrc Research Center

Also I would like to have some info on where that license comes from, searching "Dharma MIT license" doesn't seem to help me much.