Add optional callback to ChunkedInsert

This commit adds an optional `callback` argument to the `ChunkedInsert`
object. This callback is a callable object which gets called before the
chunked insert happens. This is useful for clearing any local caches
that may be in place to deal with the eventual consistency resulting
from the delayed nature of the chunked inserts.

For example,

```
cache = set()
chunked_table = ChunkedInsert(table, callback=lambda queue: cache.clear())
while True:
    data = get_data_id()
    key = data['key']
    if key in cache or table.find_one(key=key)
        continue
    cache.add(key)
    chunked_table.insert(data)
```

test/test_dataset.py

@@ -9,7 +9,7 @@ from datetime import datetime
 from sqlalchemy import FLOAT, TEXT, BIGINT
 from sqlalchemy.exc import IntegrityError, SQLAlchemyError, ArgumentError
 
-from dataset import connect
+from dataset import connect, chunked
 
 from .sample_data import TEST_DATA, TEST_CITY_1
 
@@ -394,6 +394,25 @@ class TableTestCase(unittest.TestCase):
         self.tbl.insert_many(data, chunk_size=13)
         assert len(self.tbl) == len(data) + 6
 
+    def test_chunked_insert(self):
+        data = TEST_DATA * 100
+        with  chunked.ChunkedInsert(self.tbl) as chunk_tbl:
+            for item in data:
+                chunk_tbl.insert(item)
+        assert len(self.tbl) == len(data) + 6
+
+    def test_chunked_insert_callback(self):
+        data = TEST_DATA * 100
+        N = 0
+        def callback(queue):
+            nonlocal N
+            N += len(queue)
+        with  chunked.ChunkedInsert(self.tbl, callback=callback) as chunk_tbl:
+            for item in data:
+                chunk_tbl.insert(item)
+        assert len(data) == N
+        assert len(self.tbl) == len(data) + 6
+
     def test_update_many(self):
         tbl = self.db['update_many_test']
         tbl.insert_many([