hlsenc: add use_localtime_mkdir option to automatically create time-based directory

hlsenc: add use_localtime_mkdir option to automatically create time-based directory

Use with -use_localtime, and set -hls_segment_filename to a path which
contains a subdirectory i.e. /some/path/%Y%m%d/%Y%m%dT%H%M%S-%s.ts
This will mkdir the %Y%m%d-part of the path if it does not already
exist.
In addition, each filename in the playlist output will be prefixed with
this subdirectory (if playlist and segment shares the same base path).

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>