ffplay: add -af option

Based on a patch by Stefano Sabatini <stefasab@gmail.com>:
http://ffmpeg.org/pipermail/ffmpeg-devel/2013-February/138452.htmlSigned-off-by: Marton Balint <cus@passwd.hu>

Changelog

@@ -8,6 +8,7 @@ version <next>:
   or vice versa
 - support for Monkey's Audio versions from 3.93
 - perms and aperms filters
+- audio filtering support in ffplay
 
 
 version 1.2:

doc/ffplay.texi

@@ -84,6 +84,12 @@ output. In the filter graph, the input is associated to the label
 ffmpeg-filters manual for more information about the filtergraph
 syntax.
 
+@item -af @var{filter_graph}
+@var{filter_graph} is a description of the filter graph to apply to
+the input audio.
+Use the option "-filters" to show all the available filters (including
+sources and sinks).
+
 @item -i @var{input_file}
 Read @var{input_file}.
 @end table