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

server/cli/cli.py

@@ -0,0 +1,12 @@
+import click
+
+from .launch import launch
+
+
+@click.group(name='cellxgene', context_settings=dict(max_content_width=85))
+@click.version_option(version='0.0.1', prog_name='cellxgene', message='[%(prog)s] Version %(version)s')
+def cli():
+    pass
+
+
+cli.add_command(launch)