Allow "known" alternative repo as installer source

[tavivat]

Hi,

I do have many apps installed trough F-Droid that seems considered a not trusted source, would be possible to add this as trusted source or allow to configure a list of trusted sources ?

an example error is:

[-] App "net.gaast.giggity" (1 high)                                                                                                                                                                         
high: Package "net.gaast.giggity" was manually installed by an unknown installer: "org.fdroid.fdroid" 

Thanks for the great work!