Change icon helper input params

I often want to add html options like a class to my icons, but it's not very often i want to have text. That means that i end up writing `nil` all the time (fx. `<%= icon :user, nil, class: "color-blue" %>`). 

This pull request should fix that problem, and still be backwards compatible :) 

Before:
```ruby
<%= icon :spinner, nil, class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin"></i>

<%= icon :spinner, "Loading", class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin">Loading</i>
```
After
```ruby
<%= icon :spinner, nil, class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin"></i>

<%= icon :spinner, class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin"></i>

<%= icon :spinner, "Loading", class: "fa-spin" %>
#=> <i class="fa fa-spinner fa-spin">Loading</i>
```