`import plotly` creates (possibly) unnecessary credentials

[twocs]

I was investigating an issue with the plotly/MATLAB-Online API (imatlab/imatlab#13) and found that the Python statement has an undesirable effect for offline plotly:
import plotly

Whenever this is encountered, and the files do not yet exist, the folder ~/.plotly is created and then two files ~/.plotly/.credentials and ~/.plotly/.config are also created.

This is annoying if we want to run Plotly offline because these files are unnecessary. But it is also different than plotly/MATLAB-Online, which on Windows, does not even use ~/.plotly as the plotly directory. On Windows, it uses %APPDATA%\.plotly instead.

[jonmmease]

Done in #1195. With this, the ~/.plotly directory is not created until credentials are explicitly set.