Add support for specifying a custom unaccent function

This way, you can use :ignore => [:accents] and index the resulting
expression. This is an issue because PostgreSQL (as of 9.1) does not
mark its included unaccent() function as IMMUTABLE, which makes it
ineligible for being indexed.

README.rdoc

@@ -495,6 +495,10 @@ Ignoring accents uses the {unaccent contrib package}[http://www.postgresql.org/d
   SpanishQuestion.gringo_search("Que") # => [what]
   SpanishQuestion.gringo_search("Cüåñtô") # => [how_many]
 
+Advanced users may wish to add indexes for the expressions that pg_search generates. Unfortunately, the unaccent function supplied by this contrib package is not indexable (as of PostgreSQL 9.1). Thus, you may want to write your own wrapper function and use it instead. This can be configured by calling the following code, perhaps in an initializer.
+
+  PgSearch.unaccent_function = "my_unaccent"
+
 === Using tsvector columns
 
 PostgreSQL allows you the ability to search against a column with type tsvector instead of using an expression; this speeds up searching dramatically as it offloads creation of the tsvector that the tsquery is evaluated against.

lib/pg_search.rb

@@ -18,6 +18,9 @@ module PgSearch
   mattr_accessor :multisearch_options
   self.multisearch_options = {}
 
+  mattr_accessor :unaccent_function
+  self.unaccent_function = "unaccent"
+
   module ClassMethods
     def pg_search_scope(name, options)
       scope = PgSearch::Scope.new(name, self, options)

lib/pg_search/normalizer.rb

@@ -12,7 +12,7 @@ def add_normalization(sql_expression)
             #{config.inspect}
           MESSAGE
         else
-          "unaccent(#{sql_expression})"
+          "#{PgSearch.unaccent_function}(#{sql_expression})"
         end
       else
         sql_expression

spec/pg_search/normalizer_spec.rb

@@ -10,6 +10,17 @@
           normalizer = PgSearch::Normalizer.new(config)
           normalizer.add_normalization("foo").should == "unaccent(foo)"
         end
+
+        context "when a custom unaccent function is specified" do
+          it "wraps the expression in that function" do
+            PgSearch.stub(:unaccent_function).and_return("my_unaccent")
+
+            config = stub("config", ignore: [:accents], postgresql_version: 90000)
+
+            normalizer = PgSearch::Normalizer.new(config)
+            normalizer.add_normalization("foo").should == "my_unaccent(foo)"
+          end
+        end
       end
 
       context "when config[:ignore] does not include :accents" do