Code style & fix missing period
Remove unused Dictionary
Add property 'AudioOnly' to VideoFormats
Fix comment grammar
Add check for audio & remove unneeded regex check
Fix regex for finding DASH formats
Move deserialization of formats to VideoFormat...

This also fixes the issue where playlist videos doesn't deserialize all
information as single videos does.

Also makes all VideoFormat properties read-only outside the class.
Make VideoInfo.DeserializeJson private non-static...

Move deserialization to the initializer and remove unused 'GetPlaylist'
method.