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`
aah yes. I grabbed this from 5HT's version, which also works with Elixir. I'll fix this.