Use multiple --context option instead of single option for all keys

Change-Id: I8fc93afd294ab7f1f511618dece751d4d0ffda9d
diff --git a/reclass_tools/cli.py b/reclass_tools/cli.py
index a894251..289bc29 100644
--- a/reclass_tools/cli.py
+++ b/reclass_tools/cli.py
@@ -201,7 +201,7 @@
             help=('Path to the directory where the rendered '
                   'template will be placed'))
         render_parser.add_argument(
-            '--context', '-c', dest='contexts', nargs='+',
+            '--context', '-c', dest='contexts', action='append',
             help=('YAML/JSON files with context data to render '
                   'the template'))