Add RemappingLexer.

This pattern is used heavily in Pygments for certain lexers,
particularly Lisp variants. See #43.

remap_test.go

@@ -0,0 +1,29 @@
+package chroma
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRemappingLexer(t *testing.T) {
+	var lexer Lexer = MustNewLexer(nil, Rules{
+		"root": {
+			{`\s+`, Whitespace, nil},
+			{`\w+`, Name, nil},
+		},
+	})
+	lexer = TypeRemappingLexer(lexer, TypeRemappingMap{
+		{Name, Keyword}: {"if", "else"},
+	})
+
+	it, err := lexer.Tokenise(nil, `if true then print else end`)
+	assert.NoError(t, err)
+	expected := []*Token{
+		{Keyword, "if"}, {TextWhitespace, " "}, {Name, "true"}, {TextWhitespace, " "}, {Name, "then"},
+		{TextWhitespace, " "}, {Name, "print"}, {TextWhitespace, " "}, {Keyword, "else"},
+		{TextWhitespace, " "}, {Name, "end"},
+	}
+	actual := it.Tokens()
+	assert.Equal(t, expected, actual)
+}