not enough values to unpack koala/ast/__init__.py", line 397, in prepare_pointer

Hello,

I try to test this library by feeding it a excel project but I got this error :

```
koala/ast/__init__.py", line 408, in prepare_pointer
    start, end = code.split(':OFFSET')
ValueError: not enough values to unpack (expected 2, got 1)  
```

The `code` variable is `OFFSET([84]Base!C1,1,,COUNTA([84]Base!C:C)-1)`

Can somebody help me please? Thank you very much.

