[Suggestion] Smart rename: custom output file prefix

I'd like to name my output files `[base_filename]_[layer].webp` so I need to indicate a custom prefix `[base_filename]` for my files.

I tried the `p=prefix` option but it didn't work, it must only work for actual folder paths.

By the way, the [written doc](https://github.com/GDquest/krita-batch-exporter/blob/master/batch_exporter/Manual.md) uses square brackets around options such as `[p=path]` but there shouldn't be any according to the video. Otherwise you get an error (may need another issue).