Add Workbook.set_mock_caller() method

Possibly wrap in something like `Workbook.set_mock_caller(...)`:

```
def my_macro():
    wb = Workbook.caller()
    Range('A1').value = 1

if __name__ == '__main__':
    Workbook.set_mock_caller('/path/file.xlsm')

    my_macro()
```
