Adding rule for forgotten '-r' when grepping folders

thefuck/rules/grep_recursive.py

@@ -0,0 +1,7 @@
+def match(command, settings):
+	return (command.script.startswith('grep')
+            and 'is a directory' in command.stderr.lower())
+
+
+def get_new_command(command, settings):
+    return 'grep -r {}'.format(command.script[5:])