Add default EN stopwords to the default analyzer (#381)

* Add a default list of en stopwords

* Add the default en stopword filter to the standard tokenizers

* code review feedback

src/tokenizer/stop_word_filter.rs

@@ -39,6 +39,16 @@ impl StopWordFilter {
 
         StopWordFilter { words: set }
     }
+
+    fn english() -> StopWordFilter {
+        let words: [&'static str; 33] = [
+            "a", "an", "and", "are", "as", "at", "be", "but", "by", "for", "if", "in", "into",
+            "is", "it", "no", "not", "of", "on", "or", "such", "that", "the", "their", "then",
+            "there", "these", "they", "this", "to", "was", "will", "with",
+        ];
+
+        StopWordFilter::remove(words.iter().map(|s| s.to_string()).collect())
+    }
 }
 
 pub struct StopWordFilterStream<TailTokenStream>
@@ -98,3 +108,9 @@ where
         false
     }
 }
+
+impl Default for StopWordFilter {
+    fn default() -> StopWordFilter {
+        StopWordFilter::english()
+    }
+}

src/tokenizer/tokenizer_manager.rs

@@ -8,6 +8,7 @@ use tokenizer::RawTokenizer;
 use tokenizer::RemoveLongFilter;
 use tokenizer::SimpleTokenizer;
 use tokenizer::Stemmer;
+use tokenizer::StopWordFilter;
 use tokenizer::Tokenizer;
 
 /// The tokenizer manager serves as a store for