Add rule to remove leading shell prompt literal $ (#996)

* Add rule to remove shell prompt literals $

Rule added to handle cases where the $ symbol is used in the command,
this usually happens when the command is copy pasted from a
documentation that includes the shell prompt symbol in the code blocks.

* Change files using black and flake8 style check

* Refactor tests and rule

- Refactor test for cleaner test tables
- Removed unnecessary requires_output=True option

tests/rules/test_remove_shell_prompt_literal.py

@@ -0,0 +1,38 @@
+import pytest
+from thefuck.rules.remove_shell_prompt_literal import match, get_new_command
+from thefuck.types import Command
+
+
+@pytest.fixture
+def output():
+    return "$: command not found"
+
+
+@pytest.mark.parametrize("script", ["$ cd newdir", " $ cd newdir"])
+def test_match(script, output):
+    assert match(Command(script, output))
+
+
+@pytest.mark.parametrize(
+    "command",
+    [
+        Command("$", "$: command not found"),
+        Command(" $", "$: command not found"),
+        Command("$?", "127: command not found"),
+        Command(" $?", "127: command not found"),
+        Command("", ""),
+    ],
+)
+def test_not_match(command):
+    assert not match(command)
+
+
+@pytest.mark.parametrize(
+    "script, new_command",
+    [
+        ("$ cd newdir", "cd newdir"),
+        ("$ python3 -m virtualenv env", "python3 -m virtualenv env"),
+    ],
+)
+def test_get_new_command(script, new_command, output):
+    assert get_new_command(Command(script, output)) == new_command