macro expansion

Macro expansion callback: http://ideone.com/aKLt9

Input:
==============================

    std::string input =
        "Greeting is <<hello>> world!\n"
        "Side effects are <<sideeffect>> and <<other>> vars are untouched\n"
        "Empty <<>> macros are ok, as are stray '>>' pairs.\n"
        "<<nested <<macros>> (<<hello>>?) work>>\n"
        "The order of expansion (evaluation) is _eager_: '<<<<hello>>>>' will expand to the same as '<<bye>>'\n"
        "Lastly you can do algorithmic stuff too: <<!esrever ~ni <<hello>>>>\n"
#ifdef SUPPORT_ESCAPES // bonus: escapes
        "You can escape \\<<hello>> (not expanded to '<<hello>>')\n"
        "Demonstrate how it <<avoids <\\<nesting\\>> macros>>.\n"
#endif
        ;

Output:
==============================

this is a side effect while parsing

Greeting is bye world!
Side effects are (done) and <<other>> vars are untouched
Empty <<>> macros are ok, as are stray '>>' pairs.
<<nested <<macros>> (bye?) work>>
The order of expansion (evaluation) is _eager_: 'We meet again' will expand to the same as 'We meet again'
Lastly you can do algorithmic stuff too: eyb in reverse!
You can escape <<hello>> (not expanded to 'bye')
Demonstrate how it <<avoids <<nesting>> macros>>.