Plotly

While nowhere near ready for merge I wanted to open this PR to document the progress on the backend and to discuss at what point we'll consider it "ready".

As of right now this PR lays most of the groundwork for this backend, i.e. it already supports displaying Elements, Overlays, Layouts and HoloMaps. It also already supports a wide range of Elements.

Major missing features: - [x] Support for AdjointLayouts - [x] Support for GridSpace - [x] Add style options to Element plots - [x] Issues in updating 3D plots in a HoloMap - [x] Add experimental warning - [x] Add unit tests

Here is my WIP notebook

Future work: - Missing Elements: Spread, Spikes, Path, Polygons, Box, Arrow, RGB, HSV, Bounds, VLine, HLine, Contours, Ellipse, Table - Add support for *rotation, *ticks, show_legend, show_grid plot options

该提问来源于开源项目：holoviz/holoviews