Add prefix option to asset_hash (#1949)

This allows manually changing the filename so that fiel header changes
can be reflected on the CDN. E.g. if you turn on crossOrigin serving
(CORS) the asset hash doesn't change, but the CDN cache needs to be
broken in order to pickup the new header.

middleman-core/fixtures/asset-hash-prefix/lib/middleware.rb

@@ -0,0 +1,16 @@
+class Middleware
+  def initialize(app)
+    @app = app
+  end
+
+  def call(env)
+    status, headers, response = @app.call(env)
+    body = ''
+    response.each {|part| body += part }
+    if (env["PATH_INFO"] =~ /css$/)
+      body += "\n/* Added by Rack filter */"
+      status, headers, response = Rack::Response.new(body, status, headers).finish
+    end
+    [status, headers, response]
+  end
+end