Add a handlebars helper to list dashboards by tag in Markdown

Implements issue #279.

This adds a new handlebars helper which produces a Markdown-formatted list of links to all dashboards with a given tag. This can help in building hyperlinked dashboards. 

### Example

```markdown
### Demos
{{dashboards-tagged "demo"}}
```

Produces:

```markdown
### Demos

* [Demo: Gallery](/dashboards/1/gallery)
* [Demo: Random Data](/dashboards/2/random-data)
```