Add support for parsing Extended Communities

[spakka]

Migration of this enhancement request from Bitbucket:
https://bitbucket.org/ripencc/bgpdump-hg/issues/23

Possibly add support for parsing Extended Communities.
We already parse Communities and Large Communities, it would be a useful addition.

[jaredmauch]

I did not find a rib file that had extended communities in it handy - would be good to test with something that has it to validate this, perhaps you have one handy?