Create vendor folder and fresh dist as a target only without extras including mathjax

[archmoj]

Following the previous conversation on slack,
it seems reasonable (and less confusing) to move extras/mathjax and topojson to vendor folder and start a fresh dist.

TODOs:

• figure out if all/any of the dist files we want to keep in the dist e.g. dist/README.md
• rewrite the requirejs added in Test plotly.min.js in Require.js environment #914 so that it does not depend on the old build in dist and really tests what it should test

Next steps:

• Remove mathjax from vendor and create a script to download, complete and update MathJax-v2 files from v2.3 to 2.7.x

@plotly/plotly_js

[archmoj]

https://www.npmjs.com/package/canvas

[alexcjohnson]

💃 Looks great - thanks for bringing back (and fixing) the requirejs test!