Option initial_send on Discovery

[fluffysquirrels]

When using struct mdns::discover::Discovery, the current implementation waits for mdns_query_interval before sending the first query.

With this patch, Discovery can be configured with Discovery::initial_send(..) to send out a query immediately when the response stream starts.

I left initial_send as false by default to preserve the existing behaviour, but I think true may be a better default.

[fluffysquirrels]

I've just seen PR #34, which unconditionally does the same as this PR, as in the initial request is always sent immediately.

I'm happy for either to be merged.