db.attach(alias, filepath) method, closes #113

Will also be useful for #236

tests/test_attach.py

@@ -0,0 +1,16 @@
+from sqlite_utils import Database
+
+
+def test_attach(tmpdir):
+    foo_path = str(tmpdir / "foo.db")
+    bar_path = str(tmpdir / "bar.db")
+    db = Database(foo_path)
+    with db.conn:
+        db["foo"].insert({"id": 1, "text": "foo"})
+    db2 = Database(bar_path)
+    with db2.conn:
+        db2["bar"].insert({"id": 1, "text": "bar"})
+    db.attach("bar", bar_path)
+    assert db.execute(
+        "select * from foo union all select * from bar.bar"
+    ).fetchall() == [(1, "foo"), (1, "bar")]