text: Title: Change scatter markers edgecolor and/or edgewidth 
  Body: Hi there,
  while playing with the alpha mode (alpha=0.1) I noticed that the marker have a border. Is that a feature, or is there any way to disable it?
  
  
  ![image](https://user-images.githubusercontent.com/49436733/111429412-3fc84400-86f9-11eb-9537-196aa99b59a1.png)
  
   `
  
          if df.signal_bull_week.notna().sum() > 0:
              signal_bull_week = mpf.make_addplot( df.signal_bull_week - 1 * offset,
                                          scatter=True,
                                          markersize=40,
                                          marker='^',
                                          alpha=0.1,
                                          color='black')
              add_plots.append(signal_bull_week)
  
          if df.signal_bear_week.notna().sum() > 0:
              signal_bear_week = mpf.make_addplot(df.signal_bear_week + 1 * offset,
                                           scatter=True,
                                           marker='v',
                                           markersize=40,
                                           alpha=0.1,
                                           color='black')
              add_plots.append(signal_bear_week)
  `