Implement Unicode normalization filters

Not sure how to test

filters/norm/filter.go

@@ -0,0 +1,32 @@
+package norm
+
+import (
+	"github.com/clipperhouse/jargon"
+	"github.com/clipperhouse/jargon/filters/mapper"
+	"golang.org/x/text/unicode/norm"
+)
+
+// NFC normalizes tokens into Unicode Normalization Form C
+var NFC = newFilter(norm.NFC)
+
+// NFD normalizes tokens into Unicode Normalization Form D
+var NFD = newFilter(norm.NFD)
+
+// NFKC normalizes tokens into Unicode Normalization Form KC
+var NFKC = newFilter(norm.NFKC)
+
+// NFKD normalizes tokens into Unicode Normalization Form KD
+var NFKD = newFilter(norm.NFKD)
+
+func newFilter(form norm.Form) jargon.Filter {
+	f := func(token *jargon.Token) *jargon.Token {
+		if form.IsNormalString(token.String()) {
+			return token
+		}
+
+		s := form.String(token.String())
+		return jargon.NewToken(s, true)
+	}
+
+	return mapper.NewFilter(f)
+}

go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/kljensen/snowball v0.6.0
 	github.com/spf13/afero v1.2.2
 	golang.org/x/net v0.0.0-20200324143707-d3edc9973b7e
+	golang.org/x/text v0.3.2
 )
 
 go 1.14