Add a --silence-deprecations flag

[nex3]

This flag will help mitigate the pain of deprecations for users who need a large amount of time to schedule and execute on a migration away from deprecated behavior. It should take deprecation names (as defined by #1820) and silence all instances of those deprecations. It should error if given an unknown deprecation name, and it should not support any kind of mass silencing (e.g. by Sass version).