hls: add start_number option

doc/muxers.texi

@@ -109,6 +109,7 @@ avconv -i in.nut out.m3u8
 @item -hls_time segment length in seconds
 @item -hls_list_size maximum number of playlist entries
 @item -hls_wrap number after which index wraps
+@item -start_number first number in the sequence
 @end table
 
 @anchor{image2}

libavformat/hlsenc.c

@@ -295,6 +295,7 @@ static int hls_write_trailer(struct AVFormatContext *s)
 #define OFFSET(x) offsetof(HLSContext, x)
 #define E AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption options[] = {
+    { "start_number", "first number in the sequence",            OFFSET(number),  AV_OPT_TYPE_INT,    {.i64 = 0},     0, INT_MAX, E},
     {"hls_time",      "segment length in seconds",               OFFSET(time),    AV_OPT_TYPE_FLOAT,  {.dbl = 2},     0, FLT_MAX, E},
     {"hls_list_size", "maximum number of playlist entries",      OFFSET(size),    AV_OPT_TYPE_INT,    {.i64 = 5},     0, INT_MAX, E},
     {"hls_wrap",      "number after which the index wraps",      OFFSET(wrap),    AV_OPT_TYPE_INT,    {.i64 = 0},     0, INT_MAX, E},