Fix the quality option

It's now an array of a `min` and a `max` element.

Fixes #48

index.js

@@ -29,8 +29,9 @@ module.exports = (options = {}) => input => {
 	}
 
 	if (typeof options.quality !== 'undefined') {
-		ow(options.quality, ow.number.inRange(0, 1));
-		args.push('--quality', Math.round(options.quality * 100));
+		ow(options.quality, ow.array.length(2).ofType(ow.number.inRange(0, 1)));
+		const [min, max] = options.quality;
+		args.push('--quality', `${Math.round(min * 100)}-${Math.round(max * 100)}`);
 	}
 
 	if (typeof options.dithering !== 'undefined') {

readme.md

@@ -55,8 +55,9 @@ Remove optional metadata.
 
 ##### quality
 
-Type: `number`<br>
-Values: `0...1`
+Type: `Array<min: number, max: number>`<br>
+Values: `Array<0...1, 0...1>`<br>
+Example: `[0.3, 0.5]`
 
 Instructs pngquant to use the least amount of colors required to meet or exceed
 the max quality. If conversion results in quality below the min quality the

test.js

@@ -16,7 +16,7 @@ test('support pngquant options', async t => {
 	const buffer = await fs.readFileSync(path.join(__dirname, 'fixture.png'));
 	const data = await imageminPngquant({
 		speed: 10,
-		quality: 1
+		quality: [0.8, 1]
 	})(buffer);
 	t.true(data.length > 30000);
 	t.true(isPng(data));
@@ -38,7 +38,7 @@ test('skip optimizing a non-PNG file', async t => {
 
 test('skip optimizing a fully optimized PNG', async t => {
 	const buffer = await fs.readFileSync(path.join(__dirname, 'fixture-no-compress.png'));
-	const data = await imageminPngquant({quality: 1})(buffer);
+	const data = await imageminPngquant({quality: [0.8, 1]})(buffer);
 	t.is(data.length, buffer.length);
 	t.true(isPng(data));
 });