If I set the style on any text in a `BrowserWindow` as such:

```
P {
   font-family: cursive;
}
```

...I would expect that text to take on a "cursive" look, much as it does when I apply the same style to a page loaded in Chrome Browser.

However in Electron 1.4.12 on Mac it looks like it's `serif`.

`cursive` is one of the W3C's five standard styles along with `serif`, `sans-serif`, `monospace`, and `fantasy` [1]. All of these work as expected in Electron except `cursive`.

It seems it should work just like the others. I presume it's just an omission somewhere?

Thanks!

[1] https://www.w3.org/wiki/CSS/Properties/font-family
atom/browser/web_contents_preferences.cc
int guest_instance_id = 0;
  if (web_preferences.GetInteger(options::kGuestInstanceID, &guest_instance_id))
    command_line->AppendSwitchASCII(switches::kGuestInstanceID,
                                    base::IntToString(guest_instance_id));
  
  int opener_id;
  if (web_preferences.GetInteger(options::kOpenerID, &opener_id))
    command_line->AppendSwitchASCII(switches::kOpenerID,
                                    base::IntToString(opener_id));
#if defined(OS_MACOSX)
  
  bool scroll_bounce;
  if (web_preferences.GetBoolean(options::kScrollBounce, &scroll_bounce) &&
      scroll_bounce)
    command_line->AppendSwitch(switches::kScrollBounce);
#endif
  
  const base::ListValue* args;
  if (web_preferences.GetList("commandLineSwitches", &args)) {
    for (size_t i = 0; i < args->GetSize(); ++i) {
      std::string arg;
      if (args->GetString(i, &arg) && !arg.empty())
        command_line->AppendSwitch(arg);
    }
  }
  
  std::string blink_features;
  if (web_preferences.GetString(options::kBlinkFeatures, &blink_features))
    command_line->AppendSwitchASCII(::switches::kEnableBlinkFeatures,