Support consuming binary messages

[hpihkala]

In #29 support was added for producing raw buffer messages. However, as far as I understand, support for consuming binary messages is missing. The decoder converts message values to strings by assumption before calling the callback.

There should be a configuration parameter that controls whether to interpret messages as strings or not, or alternatively, the possibility to provide a custom message decoder function.

[haio]

We can add a option to determine whether to convert message to string or not.

[haio]

The consumer support encoding option now, when set it to buffer, the message value will be a raw buffer

[hpihkala]

Works beautifully. Thanks!