Create cat_dir rule for replacing cat with ls (#823)

* Create `cat_dir` rule for replacing `cat` with `ls` when you try to run `cat` on a directory.

* Changed to string methods in response to feedback.

Added a test to make sure lines like 'cat cat' don't become 'ls ls'.

Added trailing '\n's to test cases.

thefuck/rules/cat_dir.py

@@ -0,0 +1,10 @@
+def match(command):
+    return (
+        command.script.startswith('cat') and
+        command.output.startswith('cat: ') and
+        command.output.rstrip().endswith(': Is a directory')
+    )
+
+
+def get_new_command(command):
+    return command.script.replace('cat', 'ls', 1)