Add category name attribute parameter to scribe source.

In a response to Vlad's question here: https://groups.google.com/a/cloudera.org/group/flume-user/msg/4cad5b707a24a146
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I think one thing we may do, is to start using keyword arguments to enable
users to set attribute names on their own, and use the "old names" as
defaults. This would help readability by making attributes explicitly
defined at the same level they are used at in the configuration language.

It would look something like:
scribe(1464, categoryAttr="scribe.category")
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