Generalize plugin_output to string datatype for metaschema-framework#6

The data as-is in real-world data doesn't use markup as guessed, so we
must consume the data as a long multiline string without markup and only
process it as a string to start.

Error messaging:

[ERROR] cvc-complex-type.2.4.b: The content of element 'plugin_output' is not complete. One of '{"http://www.nessus.org/cm":h1, "http://www.nessus.org/cm":h2, "http://www.nessus.org/cm":h3, "http://www.nessus.org/cm":h4, "http://www.nessus.org/cm":h5, "http://www.nessus.org/cm":h6, "http://www.nessus.org/cm":ul, "http://www.nessus.org/cm":ol, "http://www.nessus.org/cm":pre, "http://www.nessus.org/cm":hr, "http://www.nessus.org/cm":blockquote, "http://www.nessus.org/cm":p, "http://www.nessus.org/cm":table, "http://www.nessus.org/cm":img}' is expected. [file:/home/me/example.xml{2738,17}]

nessus/nessus-module.xml

@@ -136,7 +136,7 @@
             <define-field name="xref" min-occurs="0" max-occurs="unbounded" as-type="string">
                 <group-as name="see_also" in-json="ARRAY"/>
             </define-field>        
-            <define-field name="plugin_output" min-occurs="0" max-occurs="1" as-type="markup-multiline"/>
+            <define-field name="plugin_output" min-occurs="0" max-occurs="1" as-type="string"/>
             <define-field name="plugin_version" min-occurs="0" max-occurs="1" as-type="string"/>
         </model>
     </define-assembly>