[coreSubs] option to manually entering search string

apparently manual search string option is needed in subs search/download, dont ask me why but I think its to do with incorrectly named files or files that are not scanned into library.

I have left out the setting to enable this, we dont need more settings in Subtitle section, and doesnt hurt to have this listed all the time.

if anyone wants to test, https://github.com/amet/service.subtitles.opensubtitles/tree/ManualSearch has the changes