Allow range selector to work for sub-second steps

I commonly have time series dataframes with sub-second frequencies (milli/micro/nano) and I would like to use Scatter plots with rangeselector to visualize them. Issue is rangeselector's buttons can only be provided with a step included in the following:

['month', 'year', 'day', 'hour', 'minute', 'second', 'all']

I would like to be able to pass as well: nanosecond, microsecond, and millisecond.

I am not familiar with the plotly.py codebase but I am happy to open a PR provided I can have some guidance.

先追蹤 plotly.py 如何驗證或序列化 Scatter 圖的 rangeselector 按鈕，重點放在 issue 中所述的允許 step 值。透過使用 nanosecond、microsecond 和 millisecond step 測試 rangeselector，並確認產生的繪圖設定可被接受，來驗證這項變更。