First steps to configurable parser symbols

- \ and $ defined as global variables
Added gsl.escape and gsl.substitute attributes

- to modify escape char from \
- to modify substitute sequence from $(
- only affects scripts loaded after this is changed
- use -escape:x and/or -substitute:xx from command line
- see testesc.gsl

Added 'code' and 'headers' targets into Makefile

- lets us regenerate all GXL code
- lets us rebuild the gsl.h file, since we don't use Boom any more
Merge pull request #35 from hintjens/master

Added customizable escape and substitution symbols