Make get_plotlyjs public

[PhML]

From the plot docstring:

include_plotlyjs (default=True) -- If True, include the plotly.js
    source code in the output file or string.
    Set as False if your HTML file already contains a copy of the plotly.js
    library.

This option is not really useful if we can not get the library in the HTML file in first place. It is just a matter of adding get_plotlyjs in the __init__ file of the offline module.

[jonmmease]

Thanks for the suggestion @PhML, this is being added for version 3.4.0 in #1246

[jonmmease]

Implemented in #1246