Running a bokeh server with Binder

This repository demonstrates how to run a Bokeh server from within Binder. To do so, we did the following things:

Created a bokeh-app directory in the repo with a main.py file in it. This is the application that will be served. We've added the Bokeh weather example as a demo.
Installed bokeh for the viz and nbserverproxy which we'll use to direct people to the port on which Bokeh runs. See environment.yml.
Added a custom server extension (bokehserverextension.py) that will be run to direct people to the Bokeh app (which is run on a port)
Used postBuild to enable the nbserverproxy extension, then set up and enable our custom server extension for Bokeh.
Created a Binder link that uses urlpath to point users to the port on which the Bokeh server will run:
```
https://mybinder.org/v2/gh/binder-examples/bokeh/master?urlpath=/proxy/5006/bokeh-app
```

When people click on the Binder link, they should be directed to the running Bokeh app.

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