.get() method plus support for compound primary keys (#40)

* create_table now handles compound primary keys
* CLI now accepts multiple --pk for compound primary keys
* Docs for compound primary keys with CLI and Python library
* New .get() method plus documentation

Closes #36, closes #39

tests/test_get.py

@@ -1,4 +1,34 @@
 import pytest
+from sqlite_utils.db import NotFoundError
+
+
+def test_get_rowid(fresh_db):
+    dogs = fresh_db["dogs"]
+    cleo = {"name": "Cleo", "age": 4}
+    row_id = dogs.insert(cleo).last_rowid
+    assert cleo == dogs.get(row_id)
+
+
+def test_get_primary_key(fresh_db):
+    dogs = fresh_db["dogs"]
+    cleo = {"name": "Cleo", "age": 4, "id": 5}
+    last_pk = dogs.insert(cleo, pk="id").last_pk
+    assert 5 == last_pk
+    assert cleo == dogs.get(5)
+
+
+@pytest.mark.parametrize(
+    "argument,expected_msg",
+    [(100, None), (None, None), ((1, 2), "Need 1 primary key value"), ("2", None)],
+)
+def test_get_not_found(argument, expected_msg, fresh_db):
+    fresh_db["dogs"].insert(
+        {"id": 1, "name": "Cleo", "age": 4, "is_good": True}, pk="id"
+    )
+    with pytest.raises(NotFoundError) as excinfo:
+        fresh_db["dogs"].get(argument)
+    if expected_msg is not None:
+        assert expected_msg == excinfo.value.args[0]
 
 
 @pytest.mark.parametrize(