(Config) Make URL fixing optional

[OndraZizka]

The URL manipulation done by this

HtmlUtil.fixImageSourceUrls(fileContents, config);

is not always welcome.

• First, it brings in JSoup which inherently changes the output of the Markup Engine;
• Second, relative URLs to images are valid, no need to change them. And sometimes the absolute path /... is simply intended to lead to the server root, out of JBake's scope;
• Third, the resulting web may reside on multiple DNS spaces, and having full URLs requires the web to be regenerated for each.

Where it is needed is the index page(s) or anywhere the document is rendered out of it's original place. However, JBake currently uses the same content.body for all cases.

I'll make a PR.

---

What I see as potentially more useful could be image source references, something like Wicket has. Basically, the document refers to /sourceId/my-image.jpg and the framework translates that to the correct URI or URL.
That can be leveraged in many ways. Example:

"/randomImageFromGoogle/quick brown fox jumps over a lazy dog"

would make the framework fetch the image from Google and either put the URL or download and add to local images repo.

[jonbullock]

This is something I discussed with @manikmagar (in relation to #459) that image manipulation only needs to occur when content is duplicated on other URL's such as the feed or index page and a relative path has been used. We did talk about exposing a utility method to the template engines so you have more control over when the manipulation occurs as a potential solution.

I'm working on a short term fix on #459 at the moment.