added content type option to email digest

app/models/agents/email_digest_agent.rb

@@ -18,6 +18,10 @@ module Agents
 
       You can provide a `from` address for the email, or leave it blank to default to the value of `EMAIL_FROM_ADDRESS` (`#{ENV['EMAIL_FROM_ADDRESS']}`).
 
+      You can provide a `content_type` for the email and specify `text/plain` or `text/html` to be sent.
+
+      If you do not specify `content_type`, then the recipient email server will determine the correct rendering.
+
       Set `expected_receive_period_in_days` to the maximum amount of time that you'd expect to pass between Events being received by this Agent.
     MD
 
@@ -49,6 +53,7 @@ module Agents
             from: interpolated['from'],
             subject: interpolated['subject'],
             headline: interpolated['headline'],
+            content_type: interpolated['content_type'],
             groups: groups
           ).deliver_later
         end