@db.register_function decorator, closes #162

tests/test_register_function.py

@@ -0,0 +1,16 @@
+def test_register_function(fresh_db):
+    @fresh_db.register_function
+    def reverse_string(s):
+        return "".join(reversed(list(s)))
+
+    result = fresh_db.execute('select reverse_string("hello")').fetchone()[0]
+    assert result == "olleh"
+
+
+def test_register_function_multiple_arguments(fresh_db):
+    @fresh_db.register_function
+    def a_times_b_plus_c(a, b, c):
+        return a * b + c
+
+    result = fresh_db.execute("select a_times_b_plus_c(2, 3, 4)").fetchone()[0]
+    assert result == 10