metafeeds: add test cases for metafeed/announce messages

[cryptix]

go-ssb currently doesn't plan to support upgrading from pre-MF to MF support but it would be a shame to ignore feeds created with the JS stack for instance. See the metafeed details.

Rough example is this:

1. a go-ssb peer follows @p1zza
2. @p1zza starts using a new version of $client with upgrade existing capabilities
3. @p.. therefore publishes a type: 'metafeed/announce' message, together with creating the @metapizza metafeed
4. new subfeeds announced on @metapizza (like chess or indexes or what not) should now be replicated by our go-ssb peer

The changes here should be adding a new testcase to the graph package that goes through this dance and finally asserts that its subfeeds are in the hops set. So:

• create a normal (main) feed
• create a metafeed for it
• add a new Op type to announce a metafeed
• announce it
• add some subfeeds to the metafeed
• assert that hops now has all the subfeeds

To implement this functionality I'd copy this chunk and change it to unmarshal into a new type for metafeedAnnounce. To hook it up, the new index would be opened here but instead of the filterfunc, it can use the bytypes index like here just using metafeed/announce instead.