Add command to output rendered slide content.

[MichaelHackett]

I think it would be very useful to be able to view the HTML rendering of a slide for debugging layout and figuring out how to style the content appropriately. In making my first Showoff presentation, I had a fair bit of trouble figuring out how styles were being applied and how to add my own, and being able to see the HTML code is essential. I added a quick patch to dump rendered content to the log in debug (verbose) mode (which I can submit if anyone thinks it is useful), but it's not terribly convenient, because it dumps everything.

I know one could argue that you could pull the source for the slide into a separate file and process it with the target Markdown engine directly, but it is not always possible or convenient to do this with the command line tools supplied with the libraries (if any). At least, in my particular case, the Bluecloth CLI tool does not let you set flags for controlling the Discount library's processing. Yes, I could (and probably should) add that feature to Bluecloth, but even so, it would be more convenient to be able to use the Bluecloth configuration settings I have in place in my Showoff config file. (See my proposed changes in issue #194.) Plus, there are cases where Showoff is generating content to be inserted in the slide, and it isn't possible to see that output any other way.

I plan on working on this feature myself, if no one else takes it up, but I thought I would open the idea up for discussion first, or see if anyone else is up for writing it.