Provide a `NullPrecondition` for disabling arbitrary preconditions

[TravisCarden]

It may be desirable in certain cases to disable an arbitrary precondition. Symfony's Service Container (and surely others) provides the ability to do this by replacing or decorating services. Provide a NullPrecondition that clients can use with this feature without having to write new code of their own. Remember to add documentation--in the README and/or the Wiki.

• Related: https://github.com/php-tuf/composer-stager/issues/59