Add integration tests (#79)

Adds integration tests in pytest to detect breaking changes.

tests/integration/test_cli.py

@@ -0,0 +1,25 @@
+from subprocess import Popen, PIPE
+import pytest
+
+
+def run_cli(args):
+    cmd = ["python", "-m", "rfd"] + args.split()
+    p = Popen(cmd, stdout=PIPE)
+    stdout, _ = p.communicate()
+    assert p.returncode == 0
+    return stdout
+
+
+def test_version():
+    stdout = run_cli("--version")
+    assert b"rfd v" in stdout
+
+
+@pytest.mark.parametrize("args", ["", "--sort-by score"])
+def test_threads(args):
+    run_cli("threads " + args)
+
+
+@pytest.mark.parametrize("args", ["'pizza'", "'(coffee|starbucks)'"])
+def test_search(args):
+    run_cli("search " + args)