The match option is required for the email output plugin

Whilst modifying the plugin to support sendmail delivery
the requirement for the match option was removed.

The change was made in this commit:

ef28018

This causes the email plugin to fail with the following error:

undefined method `each' for nil:NilClass

Tracing back to:

/opt/logstash/logstash.jar!/logstash/outputs/email.rb:173:in `receive'"

This commit reverts that change and marks the match option as
required.

lib/logstash/outputs/email.rb

@@ -9,7 +9,7 @@ class LogStash::Outputs::Email < LogStash::Outputs::Base
 
   # the registered fields that we want to monitor
   # A hash of matches of field => value
-  config :match, :validate => :hash
+  config :match, :validate => :hash, :required => true
 
   # the To address setting - fully qualified email address to send to
   config :to, :validate => :string, :required => true