feat proposal: `@Required` decorator instead of / beside `@AllowNull`

[Ettapp]

(I'll break the formating rule for this one as it is a feat * proposal *, sorry if this causes harm)

As the columns of a table / model are by default "nullable" and as the @AllowNull decorator's purpose, when used without parameter, is to set the decorated column to be "nullable" too, I found myself using @AllowNull(false) almost everywhere, and then created a @Required decorator to fix this minor annoyance.

Before creating a pull request I wanted to be sure that I won't be annoying anybody with it, because of myself missing an obvious reason preventing this decorator to be used and merged in the repo... and also giving an early chance to propose an alternative name instead of @Required (like @NotNullable).

Thank you in advance for your reply, and a huge thanks for this lib !!