allow double backslash escape without quotes

Related #142

eval_test.go

@@ -101,6 +101,12 @@ var gEvalTests = []struct {
 		[]expr{&callExpr{"echo", []string{"hello\nworld"}, 1}},
 	},
 
+	{
+		`echo hello\\world`,
+		[]string{"echo", `hello\world`, "\n"},
+		[]expr{&callExpr{"echo", []string{`hello\world`}, 1}},
+	},
+
 	{
 		`echo hello\zworld`,
 		[]string{"echo", "helloworld", "\n"},

scan.go

@@ -293,7 +293,7 @@ scan:
 		for !s.eof && !isSpace(s.chr) && s.chr != ';' && s.chr != '#' {
 			if s.chr == '\\' {
 				s.next()
-				if isSpace(s.chr) {
+				if isSpace(s.chr) || s.chr == '\\' {
 					buf = append(buf, s.chr)
 					s.next()
 				} else {