[Feature request] adding Subplot titles

Hi,

Is there any function like the python version:
fig = tools.make_subplots(rows=2, cols=2, subplot_titles=('Plot 1', 'Plot 2','Plot 3', 'Plot 4'))
https://plot.ly/python/subplots/#simple-subplot

I spent hours today trying to figure that out, the only workaround I found is here:
https://stackoverflow.com/questions/37285729/how-to-give-subtitles-for-subplot-in-plot-ly-using-r
But it might get into trouble when your dataset is dynamic and shareX = T, which you don't know what subplot goes to which location.

首先，將所要求的 R subplot-title 行為與連結的 Python tools.make_subplots 範例，以及針對 shareX = T 的動態資料集所描述的替代方案進行比較。完成的標準是：subplot 標題能可靠地對應到其 subplot 位置，包括資料集為動態的情況，且 R API 對外提供此行為。