Disable convertStyleToAttrs plugin by default

Ref #1362 #1360

From the [spec](https://www.w3.org/TR/SVG11/styling.html#UsingPresentationAttributes):

> Presentation attributes have lower priority than other CSS style rules specified in author style sheets or ‘style’ attributes.

Though we replace inline styles with attributes without checking if
thare is any `<style>` element. This makes the plugin unsafe and it
should not be enabled by default.

plugins/convertStyleToAttrs.js

@@ -2,7 +2,7 @@
 
 exports.type = 'perItem';
 
-exports.active = true;
+exports.active = false;
 
 exports.description = 'converts style to attributes';
 

test/svgo/_index.js

@@ -61,4 +61,9 @@ describe('svgo', () => {
     const result = optimize('<svg />', { input: 'file', path: 'input.svg' });
     expect(result.data).to.equal('<svg/>');
   });
+  it('should preserve style specifity over attributes', async () => {
+    const [original, expected] = await parseFixture('style-specifity.svg');
+    const result = optimize(original, { input: 'file', path: 'input.svg', js2svg: { pretty: true } });
+    expect(normalize(result.data)).to.equal(expected);
+  });
 });