Option to name the output file(s) based on input filename structure

[johnbeech]

Raised by email request:

If I were to input the parts "heroes_01_01.png" to "heroes_01_09.png", it could automatically name the output file "heroes_01.png" or even "heroes_01_01-merge.png"

This would rely on identifying a common prefix to the files, and then reusing that common prefix in the output. Could possible use template values, such as {{common_prefix}} when specifying the output filename.