Escape [] to avoid Maruku warnings

Generating the site was producing warnings:

```
 ___________________________________________________________________________
| Maruku tells you:
+---------------------------------------------------------------------------
| Could not find ref_id = "md_im_linksource_httpsgithubcommichaelminterfractureurl_nil" for md_link([
|       md_im_link(["Source"], "https://github.com/michaelminter/fracture-url", nil)
| ],"md_im_linksource_httpsgithubcommichaelminterfractureurl_nil")
| Available refs are []
+---------------------------------------------------------------------------
!/Users/matt/.rvm/gems/ruby-1.9.3-p385/gems/maruku-0.6.1/lib/maruku/errors_management.rb:49:in `maruku_error'
!/Users/matt/.rvm/gems/ruby-1.9.3-p385/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:716:in `to_html_link'
!/Users/matt/.rvm/gems/ruby-1.9.3-p385/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:970:in `block in array_to_html'
!/Users/matt/.rvm/gems/ruby-1.9.3-p385/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:961:in `each'
!/Users/matt/.rvm/gems/ruby-1.9.3-p385/gems/maruku-0.6.1/lib/maruku/output/to_html.rb:961:in `array_to_html'
\___________________________________________________________________________
Not creating a link for ref_id = "md_im_linksource_httpsgithubcommichaelminterfractureurl_nil".
```

Escaping the `[]` chars removed these warnings (and makes the brackets appear in the html, which I think was the intent.)