prefix vs. postfix increment and decrement

The return value of $foo++ is different than ++$foo.  Likewise, $foo--
vs. --$foo.  This commit adds support for differentiating the two modes
(prefix vs. postfix).
simpler if/else construct
s/prefix/is_prefix/ to indicate the boolean-ness
Merge pull request #52 from brianphillips/pre-post-increment-decrement

prefix vs. postfix increment and decrement