fix duplicates handling, new :source & :allow_duplicates options

Fix for #3 

Introduces 2 new config options:
- `source` to specify the source field to match on, by default the *message* field
- `allow_duplicates` to deduplicate or not the source field, `true` by default

Also I'd like to revisit the semantic of the `filter_matched` call in multiline. before this PR, only event matching the pattern had `filter_matched` called on. I think all resulting merged events should have filter_matched applied on.  the match logic is really just used for bundling events together, I think the resulting events should all be considered a "match" no?