Implement preset-default plugin (#1513)

I saw complaints about `extendDefaultPlugins` api

- it cannot be used when svgo is installed globally
- it requires svgo to be installed when using svgo-loader or svgo-jsx
- it prevents using serializable config formats like json

In this diff I introduced the new plugin which is a bundle of all
default plugins.

```js
module.exports = {
  plugins: [
    'preset_default',
    // or
    {
      name: 'preset_default',
      floatPrecision: 4,
      overrides: {
        convertPathData: {
          applyTransforms: false
        }
      }
    }
  ]
}
```

plugins/plugins.js

@@ -1,5 +1,9 @@
 'use strict';
 
+// builtin presets
+exports['preset-default'] = require('./preset-default.js');
+
+// builtin plugins
 exports.addAttributesToSVGElement = require('./addAttributesToSVGElement.js');
 exports.addClassesToSVGElement = require('./addClassesToSVGElement.js');
 exports.cleanupAttrs = require('./cleanupAttrs.js');