include stash in default shell layout

As it only shows up when any stash is present, only relevant users will
see it.

The priority was chosen slightly lower than the branch name, as the
branch name is more relevant to check upon often in between repository
work while the stashes are more likely only checked between task chunks.

powerline/config_files/themes/shell/default.json

@@ -30,6 +30,10 @@
 				"function": "powerline.segments.shell.last_pipe_status",
 				"priority": 10
 			},
+			{
+				"function": "powerline.segments.common.vcs.stash",
+				"priority": 50
+			},
 			{
 				"function": "powerline.segments.common.vcs.branch",
 				"priority": 40