Add support for MID function. MID returns a specific number of characters from a text string, starting at the position you specify, based on the number of characters you specify.

MID(text, start_num, num_chars)

A list of the supported functions is available at http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/functions

This function can be implemented as a custom function. Check the following resources for more details on how to achieve that:

- http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/custom-functions
- https://github.com/telerik/xaml-sdk/tree/master/Spreadsheet/WPF/CustomFunctions