New post_scripts config and repeatable --postscript flag run a command per output, season/album folder, or run, substituting filename-template variables into pre-tokenized argv. The REST API rejects hook commands.
Closes#150
Document tvdb_api_key, tvdb_pin, tvdb_order and metadata_providers, the
--tvdb and --tvdb-order flags with their behaviour when an order does
not cover a whole series, the new REST API parameters, and the Matroska
TVDB2 tag prefixes.