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
content::WebContents* web_contents,
    const mate::Dictionary& web_preferences)
    : web_contents_(web_contents) {
  v8::Isolate* isolate = web_preferences.isolate();
  mate::Dictionary copied(isolate, web_preferences.GetHandle()->Clone());
  
  copied.Delete("embedder");
  copied.Delete("isGuest");
  copied.Delete("session");
  mate::ConvertFromV8(isolate, copied.GetHandle(), &web_preferences_);
  web_contents->SetUserData(UserDataKey(), this);
  instances_.push_back(this);
}
WebContentsPreferences::~WebContentsPreferences() {
  instances_.erase(
      std::remove(instances_.begin(), instances_.end(), this),
      instances_.end());
}
void WebContentsPreferences::Merge(const base::DictionaryValue& extend) {
  web_preferences_.MergeDictionary(&extend);
}
content::WebContents* WebContentsPreferences::GetWebContentsFromProcessID(