Change SubtitleProperties format: pickle -> JSON

This is BACKWARD INCOMPATIBLE change (i.e. old spf files won't load anymore) but it's required
because pickle leaves security holes that allow executing arbitrary code.

Now SubtitleProperties class requires a list of all available subtitle formats to be able to
correctly parse JSON input. In general this list is maintained by SubParser instances so SubParser
(and SubFormats) instantiating has been moved to a very beginning of Subconvert. Then this single
parser object is passed to all clients that might require it (most notably DataController, CLI and
[unfortunately] commandline args parser which uses SubtitleProperties class to validate a file
provided with '-p' option).
Slightly improved log prints.