Reject new lines in keys

lexer.go

diff --git a/lexer.go b/lexer.go
index f162565..7cb789b 100644
--- a/lexer.go
+++ b/lexer.go
@@ -254,9 +254,11 @@
 func (l *tomlLexer) lexKey() tomlLexStateFn {
 	l.ignore()
 	inQuotes := false
-	for r := l.next(); isKeyChar(r); r = l.next() {
+	for r := l.next(); isKeyChar(r) || r == '\n'; r = l.next() {
 		if r == '"' {
 			inQuotes = !inQuotes
+		} else if r == '\n' {
+			return l.errorf("keys cannot contain new lines")
 		} else if isSpace(r) && !inQuotes {
 			break
 		} else if !isValidBareChar(r) && !inQuotes {

lexer_test.go

diff --git a/lexer_test.go b/lexer_test.go
index 6b18a57..d1cd130 100644
--- a/lexer_test.go
+++ b/lexer_test.go
@@ -555,3 +555,9 @@
 		token{Position{1, 11}, tokenEOF, ""},
 	})
 }
+
+func TestKeyNewline(t *testing.T) {
+	testFlow(t, "a\n= 4", []token{
+		token{Position{1, 1}, tokenError, "keys cannot contain new lines"},
+	})
+}