crystal: get ip addresses via dig w/ipv4 and ipv6 support

dig.cr

def dig(hostname, ip_addresses_only = true, ipv6 = false)
  if ipv6
    io = IO::Memory.new
    Process.run("dig", [hostname, "AAAA", "+short"], output: io, shell: true)
    output = io.to_s.lines
    ip_addresses_only ? output.select { |s| s =~ /^[0-9a-f]+[:0-9a-f]+$/ } : output
  else
    io = IO::Memory.new
    Process.run("dig", [hostname, "A", "+short"], output: io, shell: true)
    output = io.to_s.lines
    ip_addresses_only ? output.select { |s| s =~ /^\d+(\.\d+){3}$/ } : output
  end
end