Expose `subtitle` in Plotly Express

[ndrezn]

Adding a subtitle to Plotly Charts is great! But a bit tedious:

fig.update_layout(title={'subtitle':{'text':"This is a subtitle"}})

Or, with magic underscores:

fig.update_layout(title_subtitle_text="This is a subtitle")

As a common addition to charts, we should expose subtitle through the Plotly Express API, e.g.:

px.line(
   ...
   subtitle="This is a subtitle"
)