Added generated variables for C++ code

Added ability to include variables in generated C++ code. Referencing a variable uses a dollar sign and a number starting at 0 `$[0-9]*`. Including the call string is still available by using `$`.

Examples: `glEnable($0)` `glDrawArrays($0, $1, $2)`

This might not be ideal because you can't use it to allocate or modify a variable.