Add support for local plotly.js/ builds to setup.py

[ayjayt]

Hi, this PR adds a --local= flag to setup.py updateplotlyjsdev so we don't need to use circleci to use our custom plotly.js w/ plotly.py.

This helps a little bit with community dev flow, I think.

Here is the docs:

plotly.py/contributing.md

Lines 237 to 250 in a8f5ad5

	### Local Repository
	If you have a local repository of `plotly.js` you'd like to try, you can run:
	```bash
	# In your plotly.js/ directory, prepare the package:
	$ npm run build
	$ npm pack
	$ mv plotly.js-*.tgz plotly.js.tgz
	# In your plotly.py/packages/python/plotly/ directory:
	$ python setup.py updateplotlyjsdev --local /path/to/your/plotly.js/
	```

Thanks!

edit: could probably be made to work with github repos too with a little fetch here and there

Code PR

This is a small addition to the build system, not a change to plotly.py as such

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[archmoj]

💃