Allow usage of fragment from old file name similar to Python slicing

[antimatter84]

On Windows, my batch renaming tool is called "Joe" (which German only, as I just saw):
http://toolsandmore.de/Central/Produkte/Software/Datei-Tools/Joe/

It allows to enter a renaming mask where one can specify fragments of the old filename and add additional elements literally or via placeholders. Example:

"cpt-picard" becomes "cpt-jean-luc-picard" with this mask: [1-4]jean-luc[4-]

• [1-4] refers to "cpt-"; 1st til 4th letters (incl)
• "jean-luc" is inserted as literal
• [4-] refers to "-picard.txt"; everything from the 4th letter til end

Such a technique is always quite handy and allows to rename quite a lot in one go.
Maybe it will find it's way into Bulky.