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`
http://maxim.livejournal.com/409763.html ... I can't translate the Ukranian. Note the extra line in the middle. Does this mean it needs to have the full licence included?

```
# Based on https://github.com/synrc/mad/blob/master/src/mad_bundle.erl
# Copyright (c) 2013 Maxim Sokhatsky, Synrc Research Center
# Modified MIT License, https://github.com/synrc/mad/blob/master/LICENSE :
# Software may only be used for the great good and the true happiness of all
# sentient beings.
```

seems sufficient?