Normalize settings for punct filter.

- Rename 'field' to 'source'
- Add 'target' for where the result is stored.
- Also added some doc comments.
- Tests pass

lib/logstash/filters/punct.rb

@@ -2,16 +2,17 @@
 require "logstash/filters/base"
 require "logstash/namespace"
 
-# The punct filter is for strip all strings but punctutions.
-#
-
+# Strip everything but punctuation from a field and store the remainder in the
+# a separate field. This is often used for fingerprinting log events.
 class LogStash::Filters::Punct < LogStash::Filters::Base
-
   config_name "punct"
   milestone 1
 
-  # The field which value is strip to punctution.
-  config :field, :validate => :string, :default => "message"
+  # The field reference to use for punctuation stripping
+  config :source, :validate => :string, :default => "message"
+
+  # The field to store the result.
+  config :target, :validate => :string, :default => "punct"
 
   public
   def register
@@ -22,11 +23,10 @@ class LogStash::Filters::Punct < LogStash::Filters::Base
   def filter(event)
     return unless filter?(event)
 
-    original_value = event[@field]
+    original_value = event[@source]
 
     # If for some reason the field is an array of values, take the first only.
     original_value = original_value.first if original_value.is_a?(Array)
-    punct = original_value.tr('A-Za-z0-9 \t','')
-    event["punct"] = punct
+    event[@target] = original_value.tr('A-Za-z0-9 \t','')
   end # def filter
 end # class LogStash::Filters::Punct