Add 'DefaultMaxlen' option to 'PublisherConfig'

[vitorarins]

Closes #27

[firminochangani]

Hey @vitorarins,
Perhaps it would interesting to extend the tests in order to cover the issue that you described in #27. cc: @m110

[amagow-ct]

Would there be any updated on this PR?
This would be a nice to have for my use case.

I am creating a message topic on runtime as topic.%s where %s is formatted at runtime. Because the library only allows setting the maxlen during init. I am not able to set the maxlen for my dynamic topic, causing memory issues in my redis store.

For the comment on extending tests @flowck, what would we be actually testing here?
The function options are getting passed to the redis publisher. Which should itself be tested in the redis-go library. Furthermore, the reason for this feature is to reduce the memory load on the store, which goes beyong the use of unit testing.
Thus, I believe it's not as useful to block this PR for the reason of having tests.