CLI refactor (#396)

* refactor cli to improve ux and enable easy incorporation of prepare as a subcommand

* switches to use click, which removes some boilerplate and gets us some improved ux for free

* changes the entry point for the cli

* changes the name of the browser option to --open and makes the default false

setup.py

@@ -25,6 +25,6 @@ setup(
     ),
     entry_points={
         "console_scripts":
-            ["cellxgene = server.app.app:main"]
+            ["cellxgene = server.cli.cli:cli"]
     }
 )