Shift time on splitting *.srt optionally (default: true)

Sometimes you need not to shift timecodes when the srt file is split, e.g. if you're implementing something like video chapters. In this pull request I added this possibility and it could be used as simple as it is:
```ruby
 SRT::File.parse(my_srt_file).split at: '00:19:24,500', shift_time: false
```