Add description to json output

Summary:
Adds a 'desc' field to json output so I can use that inside my vim
plugin.

Test Plan:
Lint a few things.  Description text shows up in vim

Reviewers: epriestley

Reviewed By: epriestley

Differential Revision: 1210

src/lint/renderer/ArcanistLintJSONRenderer.php

@@ -44,6 +44,7 @@ class ArcanistLintJSONRenderer {
           $message->getLine() - self::LINES_OF_CONTEXT,
           self::LINES_OF_CONTEXT * 2 + 1
         )),
+        'description' => $message->getDescription(),
       );
     }