improved: added missing directory error handling
improved: helpers: added script to cut movies with ffmpeg

Time to cut <FILE> is in a "<FILE>.time" or ".time" file
in format <time_start>-<time_finish>
time_start/finish is in format HH:MM:SS
improved: helpers: added script to join movies with ffmpeg

File list to join is in ".join" file by default (can be changed)
improved: helper: ffmpeg_time.py: input can be <time>-<duration>

... in addition to <start_time>-<finish_time>