Allow for configuration of a mask to didact certain pieces of data from an analytics record.

analytics/index.js

diff --git a/analytics/index.js b/analytics/index.js
index 23c57c7..ea95294 100644
--- a/analytics/index.js
+++ b/analytics/index.js
@@ -9,6 +9,18 @@
       record.apiproxy = res.proxy.name;
       record.apiproxy_revision = res.proxy.revision;
     }
+
+    if(config.mask) {
+      var maskString = config.mask.mask_string;
+      var fieldsToMask = config.mask.fields_to_mask;
+      fieldsToMask.forEach((f)=> {
+        //Only attempt to mask if the property exists.
+        if(record[f]) {
+          record[f] = maskString;
+        }
+      });
+    }
+    
     cb(null, record);
   };