Docs: Update documentation regarding the processor option/feature

[elycruz]

This feature has a multitude of uses and actually, also, enables Sass variable exports (as pointed out, further below).

Acceptance Criteria

We need to add an example of this to the main README file and additionally add details about different ways to use the processor option .

Example showing how to export sass/css variables to resulting compiled modules (hint: This is done using 'icss-util', and 'postcss' packages) which should be listed processor option docs:

rollup-plugin-sass/test/index.test.ts

Lines 290 to 315 in f4b2e64

	test('should support processor return type `Promise<{css: string, icssExport: {}, icssImport: {}}}>', async t => {
	const outputBundle = await rollup({
	input: 'test/fixtures/processor-promise/with-icss-exports.js',
	plugins: [
	sass({
	processor: (css) => new Promise((resolve, reject) => {
	const pcssRootNodeRslt = postcss.parse(css),
	extractedIcss = extractICSS(pcssRootNodeRslt, true),
	cleanedCss = pcssRootNodeRslt.toString(),
	out = Object.assign({}, extractedIcss.icssExports, {
	css: cleanedCss,
	});
	// console.table(extractedIcss);
	// console.log(out);
	resolve(out);
	}),
	options: sassOptions,
	}),
	],
	}),
	{output} = await outputBundle.generate(generateOptions),
	rslt = squash(unwrap(output));
	t.true(rslt.includes(expectA2));
	t.true(rslt.includes(expectB));
	});