Add filter reason

[Jake-Shadle]

There are multiple reasons for why any particular crate won't end up in the final graph, but the filter callback only says that a crate was filtered, not why, so in complicated scenarios it can be difficult to figure that out, we should add a reason for why every crate was filtered so additional context is available to the user.

Another option, may be to add an optional logging feature that logs out debug info at each decision point, but I'd rather avoid that.