**Is your feature request related to a problem? Please describe.**
After webContents.insertCSS() I cant find a way to remove injected stylesheet.

**Describe the solution you'd like**
insert: webContents.insertCSS('style-id', 'body { ... }') or .inserCSS('body { ... })'
remove: webContents.removeCSS('style-id') or .removeCSS() to remove all injected stylesheets


atom/renderer/api/atom_api_web_frame.cc
mate::Arguments* args,
    v8::Local<v8::Value> window,
    int world_id,
    const std::vector<mate::Dictionary>& scripts) {
  v8::Isolate* isolate = args->isolate();
  util::Promise promise(isolate);
  v8::Local<v8::Promise> handle = promise.GetHandle();
  std::vector<blink::WebScriptSource> sources;
  for (const auto& script : scripts) {
    base::string16 code;
    base::string16 url;
    int start_line = 1;
    script.Get("url", &url);
    script.Get("startLine", &start_line);
    if (!script.Get("code", &code)) {
      promise.RejectWithErrorMessage("Invalid 'code'");
      return handle;
    }
    sources.emplace_back(
        blink::WebScriptSource(blink::WebString::FromUTF16(code),
                               blink::WebURL(GURL(url)), start_line));
  }
  bool has_user_gesture = false;
  args->GetNext(&has_user_gesture);
  blink::WebLocalFrame::ScriptExecutionType scriptExecutionType =
      blink::WebLocalFrame::kSynchronous;