atee: multiply the input

usage: ... | atee [file1] [file_n] | ...
Paramters: the input will be written into the given files and forwarded
to the standard output (for further processing).
(a simple debug can be used with this)
-e {expression} {filename}: the lines for that the expression gives nonzero
  value (=true) will be stored in the given file. The input can be scattered
  into several files (one line can satisfy several expressions).

Examples:
afind *.* | atee del_log.txt | axargs del FILE
  -> delete each file (with extension) and collect the (deleted) names in
     del_log.txt

... | atee -e "strcont(line, 'apple')" apples.txt -e "strcont(line, 'grape')"
      grapes.txt | ...
  -> collect the input lines containing 'apple' in apples.txt and do similarly
     with grapes