Allow using a custom assets configuration manifest location

At the moment the only allowed path is the default one but
there could be the need to set a custom configuration manifest
location.

This commit introduces a new setting `config.assets.config_manifest`
that allows to change the location of this manifest.
This is an example configuration:

```
config.assets.config_manifest = File.expand_path('my_custom_path/manifest.js', __dir__)
```

lib/sprockets/railtie.rb

@@ -68,8 +68,8 @@ class Railtie < ::Rails::Railtie
     include Sprockets::Rails::Utils
 
     class ManifestNeededError < StandardError
-      def initialize
-        msg = "Expected to find a manifest file in `app/assets/config/manifest.js`\n" +
+      def initialize(manifest_path)
+        msg = "Expected to find a manifest file in `#{manifest_path}`\n" +
         "But did not, please create this file and use it to link any assets that need\n" +
         "to be rendered by your app:\n\n" +
         "Example:\n" +
@@ -99,11 +99,22 @@ def configure(&block)
     config.assets.prefix      = "/assets"
     config.assets.manifest    = nil
     config.assets.quiet       = false
+    config.assets.config_manifest = nil
+
+    initializer :set_config_manifest_path do |app|
+      config_manifest = if app.config.assets.config_manifest.present?
+                          Pathname.new(app.config.assets.config_manifest)
+                        else
+                          ::Rails.root.join("app/assets/config/manifest.js")
+                        end
+      raise ManifestNeededError, config_manifest unless config_manifest.exist?
+
+      app.config.assets.config_manifest = config_manifest.to_s
+    end
 
     initializer :set_default_precompile do |app|
       if using_sprockets4?
-        raise ManifestNeededError unless ::Rails.root.join("app/assets/config/manifest.js").exist?
-        app.config.assets.precompile += %w( manifest.js )
+        app.config.assets.precompile += [app.config.assets.config_manifest]
       else
         app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
       end

test/test_railtie.rb

@@ -12,6 +12,15 @@ def silence_stderr
   $stderr.reopen orig_stderr
 end
 
+def create_config_manifest(dir = "app/assets/config")
+  Dir.chdir(app.root) do
+    FileUtils.mkdir_p(dir)
+    File.open("#{ dir }/manifest.js", "w") do |f|
+      f << ""
+    end
+  end
+end
+
 class TestBoot < Minitest::Test
   include ActiveSupport::Testing::Isolation
 
@@ -39,13 +48,7 @@ def setup
     @app.config.active_support.deprecation = :notify
     ActionView::Base # load ActionView
 
-    Dir.chdir(app.root) do
-      dir = "app/assets/config"
-      FileUtils.mkdir_p(dir)
-      File.open("#{ dir }/manifest.js", "w") do |f|
-        f << ""
-      end
-    end
+    create_config_manifest
   end
 
   def test_initialize
@@ -295,6 +298,31 @@ def test_sprockets_context_helper
     assert_nil env.context_class.config.asset_host
   end
 
+  def test_default_config_manifest_path
+    app.initialize!
+    assert_match %r{app/assets/config.manifest.js$}, app.config.assets.config_manifest
+  end
+
+  def test_custom_config_manifest_path
+    create_config_manifest("my/custom/path")
+    app.configure do
+      config.assets.config_manifest = 'my/custom/path/manifest.js'
+    end
+    app.initialize!
+
+    assert_match %r{my/custom/path/manifest.js$}, app.config.assets.config_manifest
+  end
+
+  def test_config_manifest_raises_when_does_not_exist
+    app.configure do
+      config.assets.config_manifest = 'not/found/manifest.js'
+    end
+
+    assert_raises Sprockets::Railtie::ManifestNeededError do
+      app.initialize!
+    end
+  end
+
   def test_manifest_path
     app.configure do
       config.assets.manifest = Rails.root.join('config','foo','bar.json')