Add spaces_per_tab parameter to MD010/no-hard-tabs (fixes #384). (#385)

schema/build-config-schema.js

@@ -135,6 +135,11 @@ rules.forEach(function forRule(rule) {
           "description": "Include code blocks",
           "type": "boolean",
           "default": true
+        },
+        "spaces_per_tab": {
+          "description": "Number of spaces for each hard tab",
+          "type": "number",
+          "default": 1
         }
       };
       break;