Add feature to disallow user-specified ASNs from downstreams

[forkwhilefork]

Customers should never announce routes from tier1s. If they do, it's probably a leak. This may also be true of peers... though I might be able to peer with e.g. 6939, so maybe that should be configurable.

[forkwhilefork]

This should implement a new function which can be used in filters that's very similar to has_bogon_asn() but uses a different constant list of ASNs. That list will be configurable by the user.

[forkwhilefork]

This is blocked by #37.

[forkwhilefork]

This is partially implemented. There's a static constant that lists transit ASNs that should not be allowed. There is not a way to change this in the config file though.