fix(markdown): prevent unescaped objects in markdown AST

makes sure that the `type` property is either a string or an array before passing it into the
Markdown AST

fixes #201

lib/locales/en_US.json

@@ -176,6 +176,7 @@
     "the string must be a JSON Pointer, according to ": "the string must be a JSON Pointer, according to ",
     "the string must be a URI template, according to ": "the string must be a URI template, according to ",
     "the string must be a regular expression, according to ": "the string must be a regular expression, according to ",
-    "the string must be a relative JSON Pointer, according to ": "the string must be a relative JSON Pointer, according to "
+    "the string must be a relative JSON Pointer, according to ": "the string must be a relative JSON Pointer, according to ",
+    "Unknown Type": "Unknown Type"
   }
 }

lib/markdownBuilder.js

@@ -277,6 +277,9 @@ function build({
   }
 
   function type(property) {
+    if (typeof property[keyword`type`] === 'object') {
+      return text(i18n`Unknown Type`);
+    }
     const types = Array.isArray(property[keyword`type`]) ? property[keyword`type`] : [property[keyword`type`]];
     const realtypes = flist(filter(types, mytype => mytype !== 'null' && mytype !== undefined));
     if (property[keyword`allOf`] || property[keyword`anyOf`] || property[keyword`oneOf`] || property[keyword`not`]) {