Make icons compatible with Fontello

While it's harder to ask the Fontello folks to add support for Feather, it's probably better that Feather icons are made to be compatible with Fontello. At the moment, they are not.

## Steps to reproduce

- Go to http://fontello.com/
- Drag a Feather SVG file into Fontello page.

## Expected result

- See Feather icon.

## Actual result

- Empty slot.

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Fontello has a Wiki page for making SVG image to work with it here: https://github.com/fontello/fontello/wiki/How-to-use-custom-images#importing-svg-images