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.

README.md

@@ -177,6 +177,7 @@ following rules are enabled by default:
 * `aws_cli` – fixes misspelled commands like `aws dynamdb scan`;
 * `cargo` – runs `cargo build` instead of `cargo`;
 * `cargo_no_command` – fixes wrongs commands like `cargo buid`;
+* `cat_dir` – replaces `cat` with `ls` when you try to `cat` a directory
 * `cd_correction` – spellchecks and correct failed cd commands;
 * `cd_mkdir` – creates directories before cd'ing into them;
 * `cd_parent` – changes `cd..` to `cd ..`;