Add possibility to skip columns/values in CSV

Add function `batch_slice_columns` which meant to resolve issue
when CSV contains some redundant columns,
or columns not existing in corresponding Model

Usage example:

```ruby
ActiveAdmin.register Post
  active_admin_import before_batch_import: lambda { |importer|
                        importer.batch_slice_columns(['name', 'birthday'])
                      }
end
```

In this example  Post has columns 'name', 'last_name' and 'birthday'
And on import only 'name' and 'birthday' will be used in INSERT-SQL

 Changes to be committed:

Author: workgena

lib/active_admin_import/importer.rb

@@ -60,6 +60,30 @@ def batch_replace(header_key, options)
       end
     end
 
+    # Use it when CSV file contain redundant columns
+    #
+    # Example:
+    #
+    # ActiveAdmin.register Post
+    #   active_admin_import before_batch_import: lambda { |importer|
+    #                         importer.batch_slice_columns(['name', 'birthday'])
+    #                       }
+    # end
+    #
+    def batch_slice_columns(slice_columns)
+      use_indexes = []
+      headers.values.each_with_index do |val, index|
+        use_indexes << index if val.in?(slice_columns)
+      end
+      return csv_lines if use_indexes.empty?
+      # slice CSV headers
+      @headers = headers.to_a.values_at(*use_indexes).to_h
+      # slice CSV values
+      csv_lines.map! do |line|
+        line.values_at(*use_indexes)
+      end
+    end
+
     def values_at(header_key)
       csv_lines.collect { |line| line[header_index(header_key)] }.uniq
     end

spec/import_spec.rb

@@ -433,6 +433,43 @@ def upload_file!(name, ext = 'csv')
     end
   end
 
+  context "with slice_columns option" do
+    before do
+      add_author_resource template_object: ActiveAdminImport::Model.new,
+                          before_batch_import: lambda { |importer|
+                            importer.batch_slice_columns(slice_columns)
+                          }
+      visit "/admin/authors/import"
+      upload_file!(:authors)
+    end
+
+    context "slice last column and superfluous column" do
+      let(:slice_columns) { %w(name last_name not_existing_column) }
+
+      it "should not fill `birthday` column" do
+        expect(Author.pluck(:name, :last_name, :birthday)).to match_array(
+          [
+            ["Jane", "Roe", nil],
+            ["John", "Doe", nil]
+          ]
+        )
+      end
+    end
+
+    context "slice column from the middle" do
+      let(:slice_columns) { %w(name birthday) }
+
+      it "should not fill `last_name` column" do
+        expect(Author.pluck(:name, :last_name, :birthday)).to match_array(
+          [
+            ["Jane", nil, "1988-11-16".to_date],
+            ["John", nil, "1986-05-01".to_date]
+          ]
+        )
+      end
+    end
+  end
+
   context 'with invalid options' do
     let(:options) { { invalid_option: :invalid_value } }