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.

tests/rules/test_cat_dir.py

@@ -0,0 +1,29 @@
+import pytest
+from thefuck.rules.cat_dir import match, get_new_command
+from thefuck.types import Command
+
+
+@pytest.mark.parametrize('command', [
+    Command('cat foo', 'cat: foo: Is a directory\n'),
+    Command('cat /foo/bar/', 'cat: /foo/bar/: Is a directory\n'),
+    Command('cat cat/', 'cat: cat/: Is a directory\n'),
+])
+def test_match(command):
+    assert match(command)
+
+
+@pytest.mark.parametrize('command', [
+    Command('cat foo', 'foo bar baz'),
+    Command('cat foo bar', 'foo bar baz'),
+])
+def test_not_match(command):
+    assert not match(command)
+
+
+@pytest.mark.parametrize('command, new_command', [
+    (Command('cat foo', 'cat: foo: Is a directory\n'), 'ls foo'),
+    (Command('cat /foo/bar/', 'cat: /foo/bar/: Is a directory\n'), 'ls /foo/bar/'),
+    (Command('cat cat', 'cat: cat: Is a directory\n'), 'ls cat'),
+])
+def test_get_new_command(command, new_command):
+    assert get_new_command(command) == new_command