Move 'key' to 'customdata'

[cpsievert]

The 'key' attribute is specific to the R package, but we can achieve the same thing via the official 'customdata' attribute. Perhaps this could be done in a backwards compatible way by renaming 'key' to customdata (if none exists) on the R side, then change some of the client side logic to filter down by customdata instead of key, kinda like this https://github.com/plotly/dash-core-components/blob/25e8e83f0195152a9e769b128ccc21674179a2e6/src/components/Graph.react.js#L6-L40