Added 'datasette --version' support

Using http://click.pocoo.org/5/api/#click.version_option

datasette/cli.py

@@ -11,6 +11,7 @@ from .utils import (
 
 
 @click.group(cls=DefaultGroup, default='serve', default_if_no_args=True)
+@click.version_option()
 def cli():
     """
     Datasette!