text: Title: Inline format definition issue
  Body: Hi,
  
  I am using XlsxWriter to put some JSON files in tabular. I think there is a bug with inline format declaration, which can be seen below.
  
  I am using Python version 3.7.3 and XlsxWriter 1.1.8 and Excel version 12.26 with macOS Mojave 10.14.5.
  
  Here is some code that demonstrates the problem:
  
  ```python
  
  sheet.write(row_index - 1, column_index, merchant_string, {'bold': True})
  
  workbook.close()
  
  ```
  
  if I change that particular line to this it is all ok.
  
  ```python
  format = worksheet.add_format({'bold': True})
  sheet.write(row_index - 1, column_index, merchant_string, format)
  ```
  
  Here is the error message that first case creates
  
  ```
  Traceback (most recent call last):
    File "/Users/xxx/Desktop/gg/xxxx/main.py", line 91, in <module>
      workbook.close()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/workbook.py", line 304, in close
      self._store_workbook()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/workbook.py", line 646, in _store_workbook
      xml_files = packager._create_package()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/packager.py", line 135, in _create_package
      self._write_worksheet_files()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/packager.py", line 190, in _write_worksheet_files
      worksheet._assemble_xml_file()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/worksheet.py", line 3745, in _assemble_xml_file
      self._write_sheet_data()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/worksheet.py", line 5242, in _write_sheet_data
      self._write_rows()
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/worksheet.py", line 5435, in _write_rows
      self._write_cell(row_num, col_num, col_ref)
    File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/xlsxwriter/worksheet.py", line 5600, in _write_cell
      xf_index = cell.format._get_xf_index()
  AttributeError: 'dict' object has no attribute '_get_xf_index'
  
  ```
  
  This can be handled out easily but I wanted this issue to be known.
  
  -----------------------------------
  Edit:
  
  Just realized by using .add_format() method, the initialized object is instance of 'xlsxwriter.format.Format', eventhough it makes my post pointless, this in-line declaration is also very intuitive to me. Maybe considered as feature request?