Restrict Rouge formatters to values in Rouge::Formatters namespace

ff0218a added support for specifying
custom Rouge formatters, but it did not actually enforce this constraint.
For example, this is valid:

```ruby
Rouge::Formatters.const_get('CSV')
=> CSV
```

Adding the `false` parameter to `const_get` prevents this:

```ruby
Rouge::Formatters.const_get('CSV', false)
NameError: uninitialized constant Rouge::Formatters::CSV
```

lib/kramdown/converter/syntax_highlighter/rouge.rb

@@ -70,7 +70,7 @@ def self.formatter_class(opts = {})
       when Class
         formatter
       when /\A[[:upper:]][[:alnum:]_]*\z/
-        ::Rouge::Formatters.const_get(formatter)
+        ::Rouge::Formatters.const_get(formatter, false)
       else
         # Available in Rouge 2.0 or later
         ::Rouge::Formatters::HTMLLegacy