support text inside the icon and changed the README

Updated icon helper and documentation

```ruby
icon('flag', text: 'Font Awesome', id: 'my-icon', class: 'strong')
 # => <i id="my-icon" class="fa fa-flag strong"></i> Font Awesome
 ```
 
-Note: the icon helper can take a hash of options that will be passed to the content_tag helper
```ruby
icon('flag', text: 'Font Awesome', icon_text: content_tag(:em, "4", class: 'badge'), id: 'my-icon', class: 'strong')
# => <i id="my-icon" class="fa fa-flag strong"><em class="badge">4</em></i> Font Awesome
```

Note: the icon helper can take a hash of options among them:
- text: The text to be appended **after** the icon node.
- icon_text: The text to be inserter **inside** the icon node.
- any other HTML option that will be passed down to the icon like class, id, etc.