#95 expander - Input stack

[Andreas Kupries]

textutil::expander currenty manages a context stack to
temporiliy hold up output material before inserting it
into the actual stream of output.

IMHO it should also manage a stack of input material,
iallowing macros to push input onto for consumption
after the macro is complete.

This would generate an effect similar to using
[expand], but without actually opening a nested output
context.