Update icon.rb

[YoungWillem]

Hello Andy!

I am slowly figuring this GitHub thing out and am fairly new to Ruby as well, so let me know if I'm going about this the wrong way. :)

I have added the modify method to icon.rb, which implements most of the options available on the game-icons.net studio. You can now use the color presets, or add a frame, stroke, gradients, shadow, etc. to your icon.

It will also accept all the same color strings as Squib does!

Here are a few test lines you can drop into Squib to see what it does:
svg data: GameIcons.get('spiral-lollipop').modify(preset: 'candy', shape: 'star7').string, layout: 'art', width: 400, height: 400, x: 200, y: 200
svg data: GameIcons.get('chocolate-bar').modify(shape: 'rounded-square', cr: 128, bg_gradient: 'o', bg: 'light_brown', bg2: 'dark_brown', fg_gradient: '/', fg: :chocolate, fg2: :chestnut).string, layout: 'art', width: 400, height: 400, x: 200, y: 600

I hope you find it useful!

[coveralls]

Coverage decreased (-38.06%) to 61.0% when pulling 267b92c on YoungWillem:modify into f108e72 on andymeneely:master.

[andymeneely]

This is really interesting - thank you for the PR! I don't think I'm going to integrate it - I want to keep this library simple and if people want to do more custom modifications they can on their own. I'm going to go over this and document some snippets, though, so that people can adapt to their liking.