Add C-like preprocessor directives to .gsx extension.

[T-Maxxx]

... as mentioned in PR #186
I think it's a good idea to add some simple preprocessor directives for conditional compiling, "#define" for symbols (+ macro).

[MichaelHillcox]

Support for a foreach would also be nice if you looking at adding more functionality to gsc

[T-Maxxx]

Right now the script compiling routine is a bunch of inlined functions, almost completely unreadable. Some keywords like for, if, thread, self are sitting here and this is the place where foreach must be put. Doing something with script before compiling much easier as you can see in PR's code.

[D4edalus]

why this is still open: #186 (comment)

[T-Maxxx]

Because it was opened after #186 and I will try to do something later.

[D4edalus]

there was no question, it was a clarification