[Doctrine][CRUD] option to force overwrite CRUD files

    Bug fix: no
    Feature addition: yes
    Backwards compatibility break: no
    Symfony2 tests pass: yes
    Fixes the following tickets: -
    Todo: - 
    License of the code: MIT
    Documentation PR: -`

Introduce a  new option on the CRUD generation command line: --force-overwrite

With that set, the Doctrine CRUD generator command doesn't stop on already existing file and overwrite everything.

Useful when coding CRUD skeletons, testing them often.