Support specifying default renderer using PLOTLY_RENDERER env variable

[jonmmease]

Closes #1559

Support specifying the default renderer using the PLOTLY_RENDERER environment variable. This environment variable is only checked when the renderer_defaults future flag is set.

cc @nicolaskruchten

[nicolaskruchten]

2 questions:

1. should we temporarily have an environment variable to set things in v4 mode also?
2. do we need a new renderer for documentation or will defaulting to notebook_connected work for nbconverting notebooks to HTML?

[jonmmease]

1. should we temporarily have an environment variable to set things in v4 mode also?

My first reaction is that this would be a bit too magical. I'm comfortable with changing the renderer based on an external environment variable since rendering is inherently context dependent But turning on v4 mode externally would change the semantics of the code with no trace of why it changed.

2. do we need a new renderer for documentation or will defaulting to notebook_connected work for nbconverting notebooks to HTML?

notebook_connected should work fine through nbconvert. But adding a custom renderer for docs is certainly an option if we run into a need for it.

[nicolaskruchten]

For 1 I agree in general of course I just through it might make doc writing a bit smoother until the launch :) no big deal either way