[PATCH] char_filter_callback for streams

Hi!

We have line_filter_callback that can exclude certain lines. The
line_transformer can replace, add or delete characters from a line.

[[[
Creata a new callback for svn_stream_readline() that can replace, add or
delete characters from a line.

* subversion/include/svn_io.h
 (svn_stream_set_line_transformer_callback): Declare

* subversion/libsvn_subr/stream.c
  (svn_stream_set_line_transformer_callback): Sets the new callback

* subversion/tests/libsvn_subr/stream-tests.c
  (line_transformer): A callback of the new function. Removes leading
    '!' characters. Does not allocate a new line.
  (test_stream_line_transformer): tests the above callback.
]]]

Mvh
Daniel