Implemented -f flag to accept a list of resolvers

[e1abrador]

Hello,

I added to the script the -f flag that will accept a list of resolvers. Basically, it goes through the list of resolvers in the order they are provided. For each IP address, it tries the first resolver; if that fails, it tries the next one, and so on, until it either finds a successful response or exhausts the list of resolvers.