Rework consumer API

[yaziine]

From now the main entry point of the Consumer is New.
New takes a pointer to a Config in parameter, it shouldn't be nil, otherwise, an error will be returned.

Thus Serve needs no parameters. The configuration is handled by New and the brokers's addresses are managed by NewConfig.

The package documentation has been updated too.

This fixes #44.

[yaziine]

@rogpeppe 🆙

[rogpeppe]

LGTM modulo a few suggestions.

A few other cleanups that could be done:

• the HandlerConfig type isn't used by the exported API and could be unexported
• the Consumer.Handle is documented that it must be called before Serve, but ISTM that collection.handlers is only accessed within a mutex and thus it should be OK to call Handle at any time, simplifying the API a little.
• the methods on the collection type are exported, but I think are only used internally, so could maybe be unexported to make it easier to understand the code.
• it would be nice if Stop documented whether all outstanding HandleMessage calls will have completed when Stop returns (or not...)

Thanks for doing this, and sorry for the delayed review!