CI_Email::print_debugger() option to limit the type of data to be printed

(an alternative to PR #1759; partially solves issue #1742)
diff --git a/user_guide_src/source/libraries/email.rst b/user_guide_src/source/libraries/email.rst
index da3bf26..8643444 100644
--- a/user_guide_src/source/libraries/email.rst
+++ b/user_guide_src/source/libraries/email.rst
@@ -268,11 +268,21 @@
 	$this->email->attach($buffer, 'attachment', 'report.pdf', 'application/pdf');
 
 $this->email->print_debugger()
--------------------------------
+------------------------------
 
 Returns a string containing any server messages, the email headers, and
 the email messsage. Useful for debugging.
 
+You can optionally specify which parts of the message should be printed.
+Valid options are: **headers**, **subject**, **body**.
+
+Example::
+
+	// Will only print the email headers, excluding the message subject and body
+	$this->email->print_debugger(array('headers'));
+
+.. note:: By default, all of the raw data will be printed.
+
 Overriding Word Wrapping
 ========================