Update Sprite styles

[rayfranco]

I am not sure why you chose visibility:hidden over display: none in the styles but this is not working with my configuration.

Instead of breaking this on everyone's configuration, I suggest to change Sprite.spriteTemplate to a function so we can easily update Sprite.styles in our implementation.

This is an example of what we could do:

var Sprite = require('svg-sprite-loader/lib/web/sprite')

// Replace visibility:hidden by display:none
Sprite.styles.pop()
Sprite.styles.push('display:none')

var globalSprite = new Sprite()

// ...

module.exports = globalSprite

If you think a PR is necessary just let me know.