Adding new html_element utility macro

Adding new html_element utility macro in `utils.html` to create links or any other html element like this:

```
{{utils.html_element('a', 'google', href='http://google.com')}}
{{utils.html_element('a', 'google', href='http://google.com', target='_blank', class='external')}}
{{utils.html_element('img', src='http://i.imgur.com/qNIy7lH.jpg')}}
{{utils.html_element('br')}}
```

Based on @lipis macro suggested in #74, see also #80 for some background on the implementation.