Tracer mechanism for showing underlying SQL queries

* Pass a tracer= function to Database constructor
* New db.tracer() contextmanager
* Neater SQL indentation, because tracer means it could be visible now
* New db.execute() and db.executescript() methods

Closes #150

tests/test_tracer.py

@@ -0,0 +1,62 @@
+import pytest
+from sqlite_utils import Database
+
+
+def test_tracer():
+    collected = []
+    db = Database(
+        memory=True, tracer=lambda sql, params: collected.append((sql, params))
+    )
+    db["dogs"].insert({"name": "Cleopaws"})
+    db["dogs"].enable_fts(["name"])
+    db["dogs"].search("Cleopaws")
+    assert collected == [
+        ("PRAGMA recursive_triggers=on;", None),
+        ("select name from sqlite_master where type = 'view'", None),
+        ("select name from sqlite_master where type = 'table'", None),
+        ("CREATE TABLE [dogs] (\n   [name] TEXT\n);\n        ", None),
+        ("select name from sqlite_master where type = 'view'", None),
+        ("INSERT INTO [dogs] ([name]) VALUES (?);", ["Cleopaws"]),
+        ("select name from sqlite_master where type = 'view'", None),
+        (
+            "CREATE VIRTUAL TABLE [dogs_fts] USING FTS5 (\n    [name],\n    content=[dogs]\n);",
+            None,
+        ),
+        (
+            "INSERT INTO [dogs_fts] (rowid, [name])\n    SELECT rowid, [name] FROM [dogs];",
+            None,
+        ),
+        ("select name from sqlite_master where type = 'view'", None),
+        (
+            'select * from "dogs" where rowid in (\n    select rowid from [dogs_fts]\n    where [dogs_fts] match :search\n)\norder by rowid',
+            ("Cleopaws",),
+        ),
+    ]
+
+
+def test_with_tracer():
+    collected = []
+    tracer = lambda sql, params: collected.append((sql, params))
+
+    db = Database(memory=True)
+
+    db["dogs"].insert({"name": "Cleopaws"})
+    db["dogs"].enable_fts(["name"])
+
+    assert len(collected) == 0
+
+    with db.tracer(tracer):
+        db["dogs"].search("Cleopaws")
+
+    assert len(collected) == 2
+    assert collected == [
+        ("select name from sqlite_master where type = 'view'", None),
+        (
+            'select * from "dogs" where rowid in (\n    select rowid from [dogs_fts]\n    where [dogs_fts] match :search\n)\norder by rowid',
+            ("Cleopaws",),
+        ),
+    ]
+
+    # Outside the with block collected should not be appended to
+    db["dogs"].insert({"name": "Cleopaws"})
+    assert len(collected) == 2