Add a new filter unescape

Proposed in #889.

app/concerns/liquid_interpolatable.rb

@@ -185,6 +185,14 @@ def uri_expand(url, limit = 5)
       url
     end
 
+    # Unescape (basic) HTML entities in a string
+    #
+    # This currently decodes the following entities only: "'",
+    # """, "<", ">", "&", "&#dd;" and "&#xhh;".
+    def unescape(input)
+      CGI.unescapeHTML(input) rescue input
+    end
+
     # Escape a string for use in XPath expression
     def to_xpath(string)
       subs = string.to_s.scan(/\G(?:\A\z|[^"]+|[^']+)/).map { |x|

spec/concerns/liquid_interpolatable_spec.rb

@@ -38,6 +38,16 @@ def validate_options
     end
   end
 
+  describe 'unescape' do
+    let(:agent) { Agents::InterpolatableAgent.new(name: "test") }
+
+    it 'should unescape basic HTML entities' do
+      agent.interpolation_context['something'] = ''<foo> & bar''
+      agent.options['cleaned'] = '{{ something | unescape }}'
+      expect(agent.interpolated['cleaned']).to eq("'<foo> & bar'")
+    end
+  end
+
   describe 'to_xpath' do
     before do
       def @filter.to_xpath_roundtrip(string)