resolve #66
I decided to implement a simple way to pattern the name of the file when there's multiple files to be generated.

Basically on the "Texture name" holder, now you can set a pattern for multiple sprites under 2 brackets with the dollar symbol.

So for example, instead of creating "animation-0.png" , "animation-1.png", etc; now you can specify the pattern by writing in the "Texture name" holder: "animation{_$}" where "{ }" indicates the pattern and $ the index. Generating "animation_0.png", "animation_1.png", etc. If you don't specify any pattern, then the default is applied with "-i".

Note: I already tested it and it's working well on web. Maybe something is missing. If that's so, please let me know to fix it.