Option to specify CDATA wrapper for content

This will result in `options.cdata = true` when cdata is passed in as `false`, so CDATA wrapping always occurs.

Add this to the bottom of the tests to expose the bug:
```javascript
    it('should wrap elements with CDATA', function(done) {
        var feed = new RSS({
            title: 'title',
            description: 'description',
            feed_url: 'http://example.com/rss.xml',
            site_url: 'http://example.com',
            cdata: true
        });

        var expectedResult = '<?xml version="1.0" encoding="UTF-8"?>\n'+
                             '<rss xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">'+
                                 '<channel>'+
                                     '<title><![CDATA[title]]></title>'+
                                     '<description><![CDATA[description]]></description>'+
                                     '<link>http://example.com</link>'+
                                     '<generator>RSS for Node</generator>'+
                                     '<lastBuildDate>' + new Date().toUTCString() +'</lastBuildDate>'+
                                     '<atom:link href="http://example.com/rss.xml" rel="self" type="application/rss+xml"/>'+
                                 '</channel>'+
                             '</rss>';

        var result = feed.xml();

        expect(result).to.equal(expectedResult);
        done();
    });

    it('should wrap not elements with CDATA', function(done) {
        var feed = new RSS({
            title: 'title',
            description: 'description',
            feed_url: 'http://example.com/rss.xml',
            site_url: 'http://example.com',
            cdata: false
        });

        var expectedResult = '<?xml version="1.0" encoding="UTF-8"?>\n'+
                             '<rss xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">'+
                                 '<channel>'+
                                     '<title>title</title>'+
                                     '<description>description</description>'+
                                     '<link>http://example.com</link>'+
                                     '<generator>RSS for Node</generator>'+
                                     '<lastBuildDate>' + new Date().toUTCString() +'</lastBuildDate>'+
                                     '<atom:link href="http://example.com/rss.xml" rel="self" type="application/rss+xml"/>'+
                                 '</channel>'+
                             '</rss>';

        var result = feed.xml();

        expect(result).to.equal(expectedResult);
        done();
    });

```