Cannot use backslash to continue long string lines

`nasm` allows this syntax:

```
bigtext: db 'Lorem ipsem', 10, \
 'second line', 10 \
 'third line', 10, 0
```

But tinyasm gives errors:

```
Error: Missing apostrophe at line 3
Error: extra characters at end of line 3
Error: Undefined instruction ''second line', 10, \' at line 4
Error: Undefined instruction ''third line', 10, \' at line 5
```