Add option to switch back to vanilla styling

[tarwich]

In #75 we gained the ability to have our markdown look like GitHub, which was a huge win. However, it opens the door for debate on whether a user should be able to choose their style of markdown. Perhaps we should have a way of choosing the "theme" that will be applied to the markdown previewer.

[tattali]

Is the project build successfully for you in Xcode?
Because I have make it on my project https://github.com/tattali/QLAddict/blob/master/QuickLookAddict/GeneratePreviewForURL.m#L23-L29

You can select the theme with
defaults write com.sub.QuickLookAddict style NAME_OF_THEME

[tattali]

tattali@4514d6f
NAME_OF_THEME = default|github
defaults write com.fiatdev.QLMarkdown theme NAME_OF_THEME