text: Title: `vlines` (vertical lines, sharing x axis) on multiple panels
  Body: Hello Daniel, 👋
  
  Thank you for your dedication in improving the MPF library ! 📈
  I am looking for different options for the X_AXIS/GRID adjustments:
  
  - [1] : Drawing vertical line for all panels ( when passing ```**kwargs(vlines)``` to ```mpf.plot()``` it is not drawing on all panels)
  
  - [2] :  Adjusting the ```grid``` for ```x_axis``` with MPF. I seen other examples where we have to use ```axes``` . Is there a more efficient way to do that with MPF? 
  
  If i well understood what others did is, returning the ```axes``` and then calling the ```add_subplot``` method from MPL wich will force me to start again from scratch using only MPL.
  
  
  I guess I missed something, here is a part of my code:
  ```
  
  df = 'pandas dataframe with datetime index and ohlcv and RSI values'
  vls = pd.date_range(df.index.min(), df.index.max(), freq='D').tolist()
  
  ap1 = [
      mpf.make_addplot(df['RSI'], panel=1, color='royalblue')
      ] 
  
  kwargs = dict(type='candle', vlines=dict(vlines=vls, linewidths=0.5, colors=('r')))
  mpf.plot(df, addplot=ap1, **kwargs)
  ```
  (If you prefer 2 different posts i can separate each request..)
  