Here is a first attempt to get widgets into Hugo. General tests are ok on my dev environment, but I have not already written tests for widgets.

1. Create a site
2. Create a widget directory inside `/widgets`. It should look like this:
```
widgets/
└── text
    ├── layouts
    │   └── widget.html
    └── README.md
```
Note that the name `widget.html` is mandatory. *Currently the context is the content of the config parameter `widgets.[mywidgetarea].[mywidget].options`*. Variables are accessible with `.content` for a text widget like the following and as described in the config below.
```
{{- if isset . "content" -}}
  {{- .content | safeHTML -}}
{{- else -}}<pre>Here is a text widget, but there is nothing to print. Please define options.content inside every text widget in your config.</pre>
{{- end -}}
```

3. Configure your site with a `widgets` variable describing widgets inside widget areas:
```
widgets:
  sidebar:
    - type: text
      options:
        content: "<h1>IT WORKS from config</h1>"
        parser: html
  showcase:
    - type: text
      options:
        content: "Here lies a showcase."
  footer:
    - type: text
      options:
        content: "Powered by Hugo with widgets."
        foo: bar
```

4. Create a template using the `widgets` call. This can be done like this: `{{ widgets "sidebar" . }}`.
5. Create content. You can also use the widget's shortcode: `{{% widgets "showcase" %}}`
6. Build and enjoy.

- Currently the widgets' context is only the content of the config variable. We should add a wider context (easy).
- I have not studied the impact on performances.
- Else?

Fixes gohugoio#2683
See gohugoio#2535

The context inside widget templates is now:

- `.` can access all classical site variables (normally including `.Content` and so on). `.Content` refers to the actual page content content.
- `.Widget` is defined as follows:
```
type Widget struct {
	Type       string
	Params     map[string]interface{}
	Identifier string
	Weight     int
	Template   *template.Template
}
```
In particular, you can access `.Widget.Type` (name of widget type) and `.Widget.Params` (config parameters).
- `.WidgetArea` contains the widget collection in the current widget area.
- `.Site.Widgets` contains the collection of all widget areas.