Support multiline string literals in HCL

This allows multiline strings to be parsed in order to make HIL
interpolations nicer in Terraform:

```
my_complex_thing = "${merge(var.list1,
                            var.list2,
                            var.list3)}"
```

decoder_test.go

@@ -79,6 +79,12 @@ func TestDecode_interface(t *testing.T) {
 			true,
 			nil,
 		},
+		{
+			"multiline_literal.hcl",
+			false,
+			map[string]interface{}{"multiline_literal": testhelper.Unix2dos(`hello
+  world`)},
+		},
 		{
 			"multiline_no_marker.hcl",
 			true,

hcl/scanner/scanner.go

@@ -469,7 +469,7 @@ func (s *Scanner) scanString() {
 		// read character after quote
 		ch := s.next()
 
-		if ch == '\n' || ch < 0 || ch == eof {
+		if ch < 0 || ch == eof {
 			s.err("literal not terminated")
 			return
 		}

hcl/scanner/scanner_test.go

@@ -10,7 +10,7 @@ import (
 	"github.com/hashicorp/hcl/hcl/token"
 )
 
-var f100 = "ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"
+var f100 = strings.Repeat("f", 100)
 
 type tokenPair struct {
 	tok  token.Type
@@ -494,7 +494,7 @@ func TestError(t *testing.T) {
 
 	testError(t, `"`, "1:2", "literal not terminated", token.STRING)
 	testError(t, `"abc`, "1:5", "literal not terminated", token.STRING)
-	testError(t, `"abc`+"\n", "1:5", "literal not terminated", token.STRING)
+	testError(t, `"abc`+"\n", "2:1", "literal not terminated", token.STRING)
 	testError(t, `/*/`, "1:4", "comment not terminated", token.COMMENT)
 }
 

hcl/strconv/quote.go

@@ -27,9 +27,6 @@ func Unquote(s string) (t string, err error) {
 	if quote != '"' {
 		return "", ErrSyntax
 	}
-	if contains(s, '\n') {
-		return "", ErrSyntax
-	}
 
 	// Is it trivial?  Avoid allocation.
 	if !contains(s, '\\') && !contains(s, quote) && !contains(s, '$') {

hcl/strconv/quote_test.go

@@ -65,8 +65,6 @@ var misquoted = []string{
 	"`\"",
 	`"\'"`,
 	`'\"'`,
-	"\"\n\"",
-	"\"\\n\n\"",
 	"'\n'",
 	`"${"`,
 	`"${foo{}"`,

test-fixtures/multiline_literal.hcl

@@ -0,0 +1,2 @@
+multiline_literal = "hello
+  world"