whisper : token-level timestamp refactoring (#49, #120)

This turned out pretty good overall. The algorithm has been moved from
main.cpp to whisper.cpp and can be reused for all subtitles types. This
means that now you can specify the maximum length of the generated
lines. Simply provide the "-ml" argument specifying the max length in
number of characters

README.md

@@ -101,13 +101,14 @@ options:
   -ot N,    --offset-t N     time offset in milliseconds (default: 0)
   -on N,    --offset-n N     segment index offset (default: 0)
   -mc N,    --max-context N  maximum number of text context tokens to store (default: max)
+  -ml N,    --max-len N      maximum segment length in characters (default: 0)
   -wt N,    --word-thold N   word timestamp probability threshold (default: 0.010000)
   -v,       --verbose        verbose output
             --translate      translate from source language to english
   -otxt,    --output-txt     output result in a text file
   -ovtt,    --output-vtt     output result in a vtt file
   -osrt,    --output-srt     output result in a srt file
-  -owts,    --output-words   output word-level timestamps to a text file
+  -owts,    --output-words   output script for generating karaoke video
   -ps,      --print_special  print special tokens
   -pc,      --print_colors   print colors
   -nt,      --no_timestamps  do not print timestamps