Add arguments `time_off` and `duration` to transcriber

Currently the transcriber processes the whole input file. From the beginning to the end.

It would be very useful to be able to pass a start time offset and/or a duration to the transcriber.

Here is a proposal how to do it:

Add (ffmpeg's) arguments `time_off` and `duration` in `python/vosk/transcriber/cli.py` after line 46.

```python
parser.add_argument("--time_off", "-ss", default=None, type=int, help="start time offset")
parser.add_argument("--duration", "-d", default=None, type=int, help="duration")
```

Pass the arguments `time_off` and `duration` to ffmpeg in function `resample_ffmpeg` in `python/vosk/transcriber/transcriber.py` (line 115):

```python
cmd = shlex.split("ffmpeg -nostdin -loglevel quiet "
"-i \'{}\' -ar {} -ac 1 {} {} -f s16le -".format(
str(infile),
SAMPLE_RATE,
f'-ss {self.args.time_off}' if self.args.time_off is not None else '', # add this
f'-t {self.args.duration}' if self.args.duration is not None else '' # and this
))
```

The function `resample_ffmpeg_async` could be adapted similarly.