First pass at adding top level .Hugo variable

This originated with the discussion here: http://discuss.gohugo.io/t/template-variables-to-pull-hugo-version-information/419

As Hugo gains popularity and more themes are built, there is going to be needed information about the current Hugo environment to determine what the version supports. I attempted to follow the pattern in `hugolib/site.go` to add `hugolib/hugo.go`, to add a top level .Hugo variable that can be accessed from shortcodes, templates and partials.

As of right now, the version is a string constant at the top of the file that would need to be changed manually with each release, that shouldn't be too onerous. If we wanted to get fancier, maybe it could pull from the current commit / tag. I'm not sure the best way to go with that.

@spf13 I'm not quite sure how to expose that variable to the templating system, but I imagine it shouldn't be difficult.

The first thing I am using the version for is for a meta generator shortcode.
```
{{% hugo-generator %}}

// outputs
<meta name="generator" content="Hugo 0.13" />
```