Support configurable settings directory with lazy initialization

[jonmmease]

This PR makes 2 changes to our handling of the .plotly settings directory.

1. The default location of this directory is still ~/.plotly, but it can now be overridden by setting the PLOTLY_DIR environment variable before Python starts. These changes are built on this PR by @itsb: user-controllable plotly config directory #661
2. Previously this settings directory was created and initialized during the initial import of the plotly library. Now, the creation of the directory and the initialization of the configuration files is delayed until an operation is performed that needs to write to the directory. Defaults are now returned when settings are accessed and the config directory does not exist.

Change 1 provides a way to customize the location of plotly.py's settings, and makes it possible for users to create and maintain multiple profiles.

Change 2 removes the concurrency issues associated with launching many plotly processes simultaneously.

This PR addresses the following issues:

• Don't create or check permissions on .plotly config directory until it a write operation is requested #1076
• user-controllable plotly config directory #661
• import plotly creates (possibly) unnecessary credentials #728
• Allow user to set plotly directory as environment variable #533
• More flexible config / credentials #225

[jonmmease]

cc @chriddyp @Kully @cldougl . Please let me know if there's anything here you're uncomfortable with given the other contexts where plotly.py is used.

[chriddyp]

Previously this settings directory was created and initialized during the initial import of the plotly library. Now, the creation of the directory and the initialization of the configuration files is delayed until an operation is performed that needs to write to the directory.

👍 works for me. a lot of dash users have questions about this when they deploy their apps on servers with protected home directories. they aren't actually using any of the files in ~/.plotly, it just appears when dash does its own import plotly.