Add additional filters to Payload before pushing content through the API

[surrealwebs]

Hello, we recently ran into an issue where the description was not reflecting the content we needed. Currently, there is no way to modify the contents of the description without modifying the post excerpt.

I have attached a patch file that adds 2 new filters that we find very useful.

1. sailthru_payload_description is applied to $data['description'] -- Allows for the modification of the description as needed.
2. sailthru_payload_data is applied to the entire payload $data array -- Allows for one final chance to modify the payload if needed. This was added instead of adding filters to every item in the $data array.

sailthru-extra-payload-filters.patch