Add insert --truncate option

Deletes all rows in the table (if it exists) before inserting new rows.
SQLite doesn't implement a TRUNCATE TABLE statement but does optimize an
unqualified DELETE FROM.

This can be handy if you want to refresh the entire contents of a table
but a) don't have a PK (so can't use --replace), b) don't want the table
to disappear (even briefly) for other connections, and c) have to handle
records that used to exist being deleted.

Ideally the replacement of rows would appear instantaneous to other
connections by putting the DELETE + INSERT in a transaction, but this is
very difficult without breaking other code as the current transaction
handling is inconsistent and non-systematic.  There exists the
possibility for the DELETE to succeed but the INSERT to fail, leaving an
empty table.  This is not much worse, however, than the current
possibility of one chunked INSERT succeeding and being committed while
the next chunked INSERT fails, leaving a partially complete operation.

docs/python-api.rst

@@ -423,6 +423,9 @@ The function can accept an iterator or generator of rows and will commit them ac
 
 You can skip inserting any records that have a primary key that already exists using ``ignore=True``. This works with both ``.insert({...}, ignore=True)`` and ``.insert_all([...], ignore=True)``.
 
+You can delete all the existing rows in the table before inserting the new
+records using ``truncate=True``. This is useful if you want to replace the data in the table.
+
 .. _python_api_insert_replace:
 
 Insert-replacing data