insert_all(..., alter=True) works for columns introduced after first 100 records

* Insert all columns for every chunk
* Update unit test to reflect new behaviour
* Test that exception is raised
* Update documentation

Closes #139. Thanks, Simon Wiles!

docs/python-api.rst

@@ -410,7 +410,7 @@ Use it like this:
         "is_good_dog": True,
     }], pk="id", column_order=("id", "twitter", "name"))
 
-The column types used in the ``CREATE TABLE`` statement are automatically derived from the types of data in that first batch of rows. Any additional or missing columns in subsequent batches will be ignored.
+The column types used in the ``CREATE TABLE`` statement are automatically derived from the types of data in that first batch of rows. Any additional columns in subsequent batches will cause a ``sqlite3.OperationalError`` exception to be raised unless the ``alter=True`` argument is supplied, in which case the new columns will be created.
 
 The function can accept an iterator or generator of rows and will commit them according to the batch size. The default batch size is 100, but you can specify a different size using the ``batch_size`` parameter: