Add an option to downgrade library

[Former user]

Since version 1.7 introduced an bug that makes Serviio practically unusable with external subtitles (http://forum.serviio.org/viewtopic.php?f=5&t=22412), can we get a feature to down-grade to 1.6.1 with our complete library, please?

[Petr Nejedly]

Downgrade compatibility is not considered. Instead post detailed log of playing the file the linked forum topic pls.