Manual Consumer Offset Management

[eapache]

Pending issue #2, there is no way for the consumer to manually specify which offset to start at - it's always 0. There should be a way to tell the consumer to start at a specific offset.

[burke]

The two options from an API perspective seem to be:

1. Add another parameters to NewConsumer; or
2. Defer go c.fetchMessages(), so that there's an opportunity to manipulate the offset before the message fetcher starts.

I don't have an opinion on which is better.

[eapache]

The problem with whatever we do is that it's API which has to change once #2 is done, and API changes are basically verboten in go.

[eapache]

That said, 1) seems saner to me.

[eapache]

Actually, we can add a "startingOffset int64" to NewConsumer, then when #2 is done we can define FetchFromBroker as -1. That won't break compatibility.

[eapache]

Although idiomatically the startingOffset should be in a ConsumerOptions struct similar to the Config struct at http://godoc.org/github.com/ActiveState/tail - this lets us add more options in the future without breaking API compat, since unspecified fields in a struct are safely nil/0 by default