Allow setting a bounds on AsIncrement / AsDecrement

[mattbrailsford]

I want to be able to decrement a value in the database for which AsDecrement would be perfect, but I want to ensure it doesn't get decremented below zero. It would therefore be good if we could pass a bounds to these methods to ensure it can't be reduced / incremented beyond a specific level.

This should use GREATEST or LEAST in SQL Server / MySql and I believe Max and Min should work in SQLite (not sure of the others)