Add tests for exit codes

tests/test_exit_codes.py

@@ -0,0 +1,21 @@
+"""Check that the CLI returns the appropriate exit code."""
+
+import subprocess
+
+
+def test_exit_code_demo():
+    """Ensure that linting the demo returns an exit code of 1."""
+    try:
+        subprocess.check_output("proselint --demo", shell=True)
+
+    except subprocess.CalledProcessError as grepexc:
+        assert(grepexc.returncode == 1)
+
+
+def test_exit_code_version():
+    """Ensure that getting the version returns an exit code of 0."""
+    try:
+        subprocess.check_output("proselint --version", shell=True)
+
+    except subprocess.CalledProcessError:
+        assert(False)