Segmenting videos (HLS support)

Added `segments_options` to job input options, which only can contain
a `segment_time` time parameter so far.

Also added `playlist` and `segments_options` columns to the Job model and API
ouput.

This feature adds as a "post-processing" action and requires
`encoder_options` to be set. They also should prepare file to be
"segmentable". Not all files can be segmented without re-encoding.

Segmenting applies on ready `destination_file`.
It generates playlist named "destination_file_basename.m3u8" and a set
of segments in a mpegts format named "destination_file_basename-%06d.ts"

Also method `Job#finalize` refactored a bit. Moved few guard clauses to
the top instead of 3-levels if-else conditions.

Related to #31
Update README with segmenting job description
Updated README, added segmenting-only job.
Merge pull request #34 from madebyhiro/pr/33

HLS jobs