Geoplot

This is a GitHub-only release with no code changes w.r.t. version `0.3.2`. This release was necessitated by the [Zenedo](https://zenodo.org/) [DOI creation workflow](https://guides.github.com/activities/citable-code/).

This release brings a revamped test suite and a variety of bug fixes to `geoplot`, as well as one breaking change:

* The default behavior of the `hue` variable has changed. Previously, a categorical colormap with `k=5` was the default. Now, a continuous colormap with `k=None` is the default.

This is an incremental release that fixes some bugs and adds some new features to `geoplot`.

* The default extent determination code has been improved.
* A plot type, `webmap`, and henceforth a new dependency, `contextily` (169).
* A new projection type, `WebMercator`, usable but all plot types but especially relevant to the new `webmap` plot type.
* All plots that support the `hue` visual parameter now additionally support a `norm` parameter for performing colormap normalization (168).
* Inputting a `Series` or `GeoSeries` to a parameter that accepts flexible types now correctly uses the input data's index (165).

Also, while this is now a code change, `geoplot` now has an official Gitter room! [Go there to ask questions about working with or developing on this library.](https://gitter.im/geoplot/community)

This release is the first comprehensive overhaul of `geoplot` since its initial design and release in early 2017. While this release adds no new "large-scale" features, you can except a much more polished user experience with `0.3.0` than previously existed.

Breaking changes include (highlights in bold):
* **The `categorical` argument has been removed. `geoplot` will now infer when a column is categorical automatically.**
* The overly complicated `aggplot` plot type has been removed and replaced with the simplified `quadtree`. Most parameters are the same, but the convex hull and auto-choropleth features have been removed.
* Plots now use the center of the `GeoDataFrame` bounding box geometry (via `GeoDataFrame.total_bounds`) instead of the cumulative average of the individual geometries, which is both faster and more accurate.
* The order of arguments for the `extent` parameter has changed. Previously it took a tuple of `(xmin, xmax, ymin, ymax)`; now it takes a tuple of `(xmin, ymin, xmax, ymax)`, for compatibility with `GeoDataFrame.total_bounds`.
* **When overplotting multiple plots on a single axis, the projection now need only be specified once, instead of once per plot function.**
* The `sankey` has been reworked. `path`, `from`, and `to` have been removed; line geometries are now read from the `GeoDataFrame` instead of transformed just-in-time. This brings `sankey` more in line with the rest of the plots in the `geoplot`.
* The `trace` and `trace_kwargs` parameters have been removed from `cartogram`. To add a trace to a `cartogram` plot, overplot a `polyplot` instead.
* **`cmap` now defaults to `viridis` (as in `matplotlib`) instead of `Set1`.**
* The minimum Python version has been raised from 3.5 to 3.6.
* `polyplot` now defaults to `zorder=-1` (e.g. "bottom of the plot").
* `kdeplot` now defaults to `shade_lowest=False`.
* The `vmin` and `vmax` parameters have been removed.

Other changes include:
* A number of edge cases have been addressed and reliability has been improved, particularly in the case of the `voronoi` and `quadtree` plot types.
* Certain common bad combinations of parameters will now raise helpful error messages instead of failing inexplicably or doing the wrong thing.
* **The documentation has been completely rewritten for ease of use and clarity.**
* Certain parameters like `clip` now accept more types of inputs.
* Code has been refactored with new `Plot` and `{Feature}Mixin` classes, which greatly increases code reuse and extensibility and will make adding new plot types in the future much easier.