Add an option to specify a file with domain names as a source of queries

[ameshkov]

Currently, godnsbench can only randomize a domain name which is nice, but it's not the same as a real server load.

Another simple option would be to allow specifying an input file with the list of domain names, godnsbench will send A query to these domain names.

-c in this case no more makes sense since the number of queries is controlled by the file contents.