Expose `convertNameToId` function as option

Allows you to define a custom function to transform the file name into the symbol id. 
A specific use case: if you create an icon set in Adobe Illustrator by having a single .AI with multiple artboards, saving them as SVG results in files with the pattern `illustratorFileName_artboardName.svg`. In such cases it is useful to be able to strip the `illustratorFileName_` prefix by writing a custom `convertNameToId` function.