Dynamically reallocate macro expansion buffer (ticket #45)

Dynamically reallocate macro expansion buffer (ticket #45)
- Eliminate the underlying limitation of macro expansion limit by
  growing the buffer as needed when adding characters to it. This
  doesn't fix the entire issue yet however: expandU() and expandMacros()
  are still limited to caller-specified buffer size, and output
  from lua-macros isn't dynamically resized. One step at a time...