Add ability to run only if previous script succeeded

[natemccurdy]

Enhancement

It would be good to have an option that will tell update_kodi to only run if the previous extension script succeeded.

This is useful in a scenario where something like VideoSort needs to sort and place files before Kodi should be updated.

Possible solution

1. Add documentation that tells the user to put update_kodi after the required extension script in NZBGet.
2. Create a yes/no option that, when yes, will check that the value of NZBPP_SCRIPTSTATUS is SUCCESS before trying to update Kodi. If it's not, we exit with code 95 (skip).