Add tests against missing fields in prmon txt and json outputs

[quantum-shift]

test_fields.cpp is added which runs prmon and parses the prmon.txt and prmon.json_snapshot files, and checks that the expected fields are present. It takes options --disable mon similar to prmon which allows testing of fields when certain monitors are disabled.

A function is added to the monitors which returns the parameter list for the monitor. This is used to get the parameters for each monitor.

Closes #152.

[graeme-a-stewart]

Looks great, thanks @quantum-shift. I have one suggestion that makes get_all_registered() easier to understand (at least for me).

[graeme-a-stewart]

I don't like this block of code as it's so asymetric. How about getting the second monitor list and using insert, like this:

// Return all registered monitors, regardless of template type
// In practice this means combining monitors with no constructor
// arguments with those (=netmon) which take a list of strings
const std::vector<std::string> get_all_registered() {
  // Standard monitors
  auto registered_monitors = registry::Registry<Imonitor>::list_registered();
  // Special monitors
  auto special_monitors =
      registry::Registry<Imonitor, std::vector<std::string>>::list_registered();
  // Merge
  registered_monitors.insert(registered_monitors.end(),
                             special_monitors.begin(), special_monitors.end());
  return registered_monitors;
}