New method to check input

The argument that is passed to NSCheck now gets passed on to a method
called "get_domains".
If the argument has .txt apended to it, the method tries to read domain
names out of the file. If it's anything besides a .txt extension, it
assumes it's a domain name and does a lookup of it. Returns an array of
domain(s). If there is an error, the get_domains method prints the
error and exits out.