Improve support of subplots

[thomas-saigre]

Figure with subplots does not raise error when exported, but the subplots are merged in a single one.

For instance

from plotly.subplots import make_subplots
import plotly.graph_objects as go

fig = make_subplots(rows=1, cols=2)

fig.add_trace(
    go.Scatter(x=[1, 2, 3], y=[4, 5, 6]),
    row=1, col=1
)

fig.add_trace(
    go.Scatter(x=[20, 30, 40], y=[50, 60, 70]),
    row=1, col=2
)

fig.update_layout(height=600, width=800, title_text="Side By Side Subplots")

results in