[added] Ability to specify style for the modal contents

This makes it so you can pass a style prop to the ReactModal component
which will be added to the contents of the modal.  This is really
nice for instances where adding a className isn't appropriate such
as when position needs to be set dynamically.

lib/components/ModalPortal.js

@@ -147,6 +147,7 @@ var ModalPortal = module.exports = React.createClass({
       },
         div({
           ref: "content",
+          style: this.props.style,
           className: cx(this.buildClassName('content'), this.props.className),
           tabIndex: "-1",
           onClick: stopPropagation,

specs/Modal.spec.js

@@ -110,6 +110,11 @@ describe('Modal', function () {
     unmountModal();
   });
 
+  it('supports adding style to the modal contents', function () {
+    var modal = renderModal({isOpen: true, style: {width: '20px'}});
+    equal(modal.portal.refs.content.getDOMNode().style.width, '20px');
+  });
+
   it('adds --after-open for animations', function() {
     var modal = renderModal({isOpen: true});
     var overlay = document.querySelector('.ReactModal__Overlay');