Add README, list aggregation, explicit reject (Juniper)

[jthurman42]

I found this script to be extremely useful, thanks a lot for opening it up! I've added a brief README with examples, and a few functional changes.

• Add an explicit 'then reject' for the Juniper filter generation
• Add an option to aggregate the prefixes fetched, and allow prefixes from the aggregate upto the specified CIDR mask (currently only used with Juniper filters)
• Ability to auto-generate the filter output name based on the ASN / AS-SET name, then appending the specified filter name as well. This could be extended to allow prefixing the name, or dropping it all together.

All of these changes have been tested, and used to generate filters that have been put into production.

[dotwaffle]

Excellent work, thank you!

Two quick suggestions before merge:

1. In commit 0993e9e you put a "reject" at the end. What if someone wanted to have a policy afterwards matching other prefixes? I'd much rather the reject be left out if that's ok? Happy to be convinced otherwise!
2. The reason this is pretty much unmaintained is there is a far better alternative now ;) Seen this? http://snar.spb.ru/prog/bgpq3/