Allow setting keep explicitly

The number of backup copies of an asset that are kept after running `assets:clean` is currently hardcoded at 2. You currently cannot configure this number. 

Instead of pushing the ability to change this via an ENV var sstephenson/sprockets#461 it was suggested that this ability be done at the rake level which results in a clean API.

So if you do not want to keep any copies of assets you can run

```sh
$ rake assets:clean[0]
```

You can keep 1 copy of assets by running:

```sh
$ rake assets:clean[1]
```

If no value is provided 2 copies will be stored.

ATP

Author: schneems

CHANGELOG.md

@@ -0,0 +1,4 @@
+*   Allow keep value to be specified for `assets:clean` run with args
+    for example `assets:clean[0]` will keep no old asset copies
+
+    *Richard Schneeman*

lib/sprockets/rails/task.rb

@@ -62,7 +62,8 @@ def define
           end
 
           desc "Remove old compiled assets"
-          task :clean => :environment do
+          task :clean, [:keep] => :environment do |t, args|
+            keep = Integer(args.keep || 2)
             with_logger do
               manifest.clean(keep)
             end

test/fixtures/foo.js

@@ -1 +1 @@
-var Foo;
+var Foo;

test/test_task.rb

@@ -27,7 +27,7 @@ def setup
     Sprockets::Rails::Task.new do |t|
       t.environment = @assets
       t.manifest    = @manifest
-      t.assets      = ['foo.js']
+      t.assets      = ['foo.js', 'foo-modified.js']
       t.log_level   = :fatal
     end
   end
@@ -77,4 +77,38 @@ def test_clean
     @rake['assets:clean'].invoke
     assert File.exist?("#{@dir}/#{digest_path}")
   end
+
+  def test_clean_with_keep_specified
+    assert !@environment_ran
+    path     = @assets['foo.js'].pathname
+    new_path = path.join("../foo-modified.js")
+
+    FileUtils.cp(path, new_path)
+
+    assert File.exist?(new_path)
+    digest_path = @assets['foo-modified.js'].digest_path
+
+    @rake['assets:precompile'].invoke
+    assert File.exist?("#{@dir}/#{digest_path}")
+    assert @environment_ran
+
+    # clean environment
+    setup
+
+    # modify file
+    File.open(new_path, "a") {|f| f.write("var Bar;") }
+    @rake['assets:precompile'].invoke
+    old_digest_path = digest_path
+    digest_path     = @assets['foo-modified.js'].digest_path
+
+    refute_equal old_digest_path, digest_path
+    assert File.exist?("#{@dir}/#{old_digest_path}")
+    assert File.exist?("#{@dir}/#{digest_path}")
+
+    @rake['assets:clean'].invoke(0)
+    assert File.exist?("#{@dir}/#{digest_path}")
+    refute File.exist?("#{@dir}/#{old_digest_path}")
+  ensure
+    FileUtils.rm(new_path) if new_path
+  end
 end