fix(Markdown): render markdown in more secure way

This commit changes markdown sanitization behaviour in following way:

class, style and data-* attributes are removed by default. These attributes
open possible vulnerability vectors to attackers.

The original behavior of sanitizer (before this commit) can be enabled by *useUnsafeMarkdown* configuration option.
Use this configuration option with caution and only in cases when you know
what you're doing.

src/core/components/primitive-model.jsx

@@ -34,7 +34,7 @@ export default class Primitive extends Component {
     let properties = schema
       .filter( ( v, key) => ["enum", "type", "format", "description", "$$ref"].indexOf(key) === -1 )
       .filterNot( (v, key) => extensions.has(key) )
-    const Markdown = getComponent("Markdown")
+    const Markdown = getComponent("Markdown", true)
     const EnumModel = getComponent("EnumModel")
     const Property = getComponent("Property")