[downloads-page] make Version available so generated links can include version

I am placing my generated downloads page in the top level of my github repo but the downloadable archives are in a `subdir/version/` format.  

This small patch makes `{{.Version}} ` available for use in your template when generating the hyperlinks.

This is my template I am using now. 
```
  "TaskSettings": {
    "downloads-page": {
      "filename": "../../downloads.md",
      "outputFormat": "by-file-extension",
      "templateExtraVars": {
        "footer": "Generated by [goxc](https://github.com/laher/goxc)"
      },
      "templateFile": "",
      "templateText": "# Downloads\n\n{{.AppName}} downloads (version {{.Version}})\n\n{{range $k, $v := .Categories}}## {{$k}}\n\n{{range $v}} * [{{.Text}}](downloads/{{.Version}}/{{.RelativeLink}})\n{{end}}\n{{end}}\n\n{{.ExtraVars.footer}}"
    }
  }
}
```