don’t override :custom if already set

Sometimes Sprockets needs to access
`options[:custom][:sprockets_context]`. If this is set, it should not be
overridden. This happens when building a LivingStyleGuide file when the
Sass source uses `asset-path`, `image-url` or similar functions.

middleman-core/lib/middleman-core/renderers/sass.rb

@@ -105,7 +105,7 @@ module Middleman
             filename: eval_file,
             line: line,
             syntax: syntax,
-            custom: { middleman_context: ctx.app }
+            custom: (options[:custom] || {}).merge(middleman_context: ctx.app)
           }
 
           if ctx.is_a?(::Middleman::TemplateContext) && file