afilter: filter the input file names with masks and forward the
remaining ones. The masks are of the same form as in afind - as well as the
positive and negative filters.

Usage: ... | afilter *.bak | ...

Examples:
afind *.* | acopy /work -fwi | afilter *.txt | acopy /texts
  -> copy everything to '/work' and then copy the text files to
     an other directory, too

afind *.* | acopy /work -fwo | afilter *.txt | atext -ins end "C++ Builder"
  -> copy everything to '/work' and then append the text to the text files
     in '/work'