Use defaultProps instead of adding props to the svg

[lencioni]

Currently, when transforming an SVG file that has attributes on the
top-level <svg> element, the transformed component ends up looking
something like this:

_extends({
  xmlns: 'http://www.w3.org/2000/svg',
  viewBox: '0 0 1000 1000'
}, props),

this adds the overhead of _extends. I think it might be a better to
instead move these values into defaultProps on the generated
component.

Here's the diff of the result:

diff --git a/master.js b/default-props.js
index cb187ed..e62cca0 100644
--- a/master.js
+++ b/default-props.js
@@ -9,8 +9,6 @@ exports.MyClassIcon = undefined;
 
 var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();
 
-var _extends = Object.assign || function (target) { for (var i = 1; i < arguments.length; i++) { var source = arguments[i]; for (var key in source) { if (Object.prototype.hasOwnProperty.call(source, key)) { target[key] = source[key]; } } } return target; };
-
 exports.MyFunctionIcon = MyFunctionIcon;
 
 var _react = require('react');
@@ -29,12 +27,7 @@ var MySvg = function () {
   function MySvg(props) {
     return _react2['default'].createElement(
       'svg',
-      _extends({
-        dataName: 'Livello 1',
-        id: 'Livello_1',
-        viewBox: '0 0 151.57 151.57',
-        xmlns: 'http://www.w3.org/2000/svg'
-      }, props),
+      props,
       _react2['default'].createElement('title', null),
       _react2['default'].createElement('circle', {
         cx: '1038.5',
@@ -78,6 +71,12 @@ var MySvg = function () {
   return MySvg;
 }();
 
+MySvg.defaultProps = {
+  dataName: 'Livello 1',
+  id: 'Livello_1',
+  viewBox: '0 0 151.57 151.57',
+  xmlns: 'http://www.w3.org/2000/svg'
+};
 function MyFunctionIcon() {
   return _react2['default'].createElement(MySvg, null);
 }

Closes #7