Save codepoints based on filename not sort order to prevent overwrite
I am facing a problem which I've mentioned in a comment on a PR from a little while back, hopefully there is a quick solve for this. Basically, the codepoints should be saved based on filename and not sort order. Otherwise, the entire set of icons change their codepoint number if an icon goes higher up the list.

**Example here:**
https://github.com/jeerbl/webfonts-loader/pull/56#issuecomment-457437039

Thank you for all the work you've put into it!