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`
ETOOIMPATIENT

-----Original Message-----
From: Loïc Hoguin <notifications@github.com>
Reply: ninenines/erlang.mk <reply@reply.github.com>>
Date: 24. November 2014 at 11:16:51
To: ninenines/erlang.mk <erlang.mk@noreply.github.com>>
Cc: Dave Cottlehuber <dch@jsonified.com>>
Subject:  Re: [erlang.mk] add escript support via plugin (#151)

> > ESCRIPT_COMMENT

A+, Dave
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