ffprobe: implement -i FILE option

Useful for mimicking the ffmpeg -i FILE syntax.

doc/ffprobe.texi

@@ -108,6 +108,9 @@ multimedia stream.
 Each media stream information is printed within a dedicated section
 with name "STREAM".
 
+@item -i @var{input_file}
+Read @var{input_file}.
+
 @end table
 @c man end
 

ffprobe.c

@@ -385,6 +385,7 @@ static const OptionDef options[] = {
     { "show_packets", OPT_BOOL, {(void*)&do_show_packets}, "show packets info" },
     { "show_streams", OPT_BOOL, {(void*)&do_show_streams}, "show streams info" },
     { "default", HAS_ARG | OPT_AUDIO | OPT_VIDEO | OPT_EXPERT, {(void*)opt_default}, "generic catch all option", "" },
+    { "i", HAS_ARG, {(void *)opt_input_file}, "read specified file", "input_file"},
     { NULL, },
 };