With FFmpeg 0.9, the -newsubtitle option was removed. If you want to add all input files' video/audio/subtitle streams to the output, use the -map options, e.g.:
ffmpeg -i movie_input.m4v -i subtitles.srt -c:v copy -c:a copy -c:s mov_text \
-map 0 -map 1 out.m4v
This will copy video and audio streams, but encode the subtitles to mov_text, which is the only officially supported subtitle format for MP4. SRT by default won't work.
The map options here specify that all the streams from the first file (0) and the second file (1) will be copied, so it'd work even if your original had multiple audio streams, for example.
For a detailed article on how to use the map option, see the FFmpeg wiki.
Reason for removing those options from the Changelog:
-newvideo/-newaudio/-newsubtitle options were removed. Not only were they
irregular and highly confusing, they were also redundant. In avconv the -map
option will create new streams in the output file and map input streams to
them. E.g. avconv -i INPUT -map 0 OUTPUT will create an output stream for
each stream in the first input file.