Clone SVGO opts object before mutating it

[cpmsmith]

Currently, when adding the essential SVGO plugins, react-svg-core directly mutates the opts object it's passed, which can cause problems if you're reusing the same options in multiple places, e.g.:

const SVGOOptions = {
	plugins: [
		{ removeTitle: true },
		{ removeStyleElement: false }
	],
	floatPrecision: 2
};

// later, in webpack.config
const output = {
	// [...]
	module: {
		rules: [
			{
				test: [/\.svg$/],
				issuer: [/\.jsx$/],
				use: [{
					loader: 'react-svg-loader',
					options: { svgo: SVGOOptions }
				}]
			}, {
				test: [/\.svg$/],
				issuer: [/\.scss$/],
				use: [{
					loader: 'image-webpack-loader',
					options: { svgo: SVGOOptions }
				}]
			}
		]
	}
	// [...]
};

When react-svg-loader is used first, it causes image-webpack-loader to apply its essential SVGO plugins. This PR prevents this.