output: add support for sprintf templating in message_properties

[yaauie]

Several Issues and Pull-Requests have been opened concerning the ability to set a message's priority from the content of the message; this PR solves that in such a way that all message_properties can support sprintf templates without any unnecessary per-event overhead.

1. at plugin registration, properties are split into constant and variable subsets.
2. per event, only the variable properties (if any) are built from the event

Resolves: #8
Duplicate of: logstash-plugins/logstash-output-rabbitmq#72
Duplicate of: logstash-plugins/logstash-output-rabbitmq#80

[andsel]

Nice @yaauie I don't know if could be useful to cite this ability also in the documentation.

[theyough]

Any update ? When do you plan to release it ?

[andsel]

LGTM

[theyough]

@yaauie Could you fix conflict, and merge it please ? thank you for your help