sql-insert, sql-update, sql-delete into join

In NHibernate hbm mappings, we can insert `<sql-insert>`, `<sql-update>` or `<sql-delete>` inside `<join>` element and it works as expected (custom insert, update or delete for joined parts). Currently Fluent NHibernate ignores those elements for joins when generates the mapping. Currently in my work I required a custom insert for a joined table because that table was shared with other entities, those entities using other columns from this shared table and I needed to override the insert statement and convert insert into a merge statement (SQL Server), because an empty row in the joined table could exist and nhibernate tried an insert statement throwing an fk exception every time. For legacy database schemas those elements are very useful and powerful.