Add a Whitelist feature

[GERShark8209]

In my game I wish to use polymod for modding support, but I don't want to pose the risk of people accessing stuff I don't want them to

But manually adding every class could get tedious if I add new ones

so perhaps there could be some sort of whitelist mode that prevents accessing any class besides specific allowed classes

[realvirtu]

I like this, but also think that blacklisting imports should be recursive