Wrap icon text in a span with alignment classes

Instead of joining the text in a way that cannot be targeted with CSS, we can wrap it in a span so that we can target it and style it however one might want!

Examples from the updated README:

```ruby
fa_icon "camera-retro"
# => <i class="fa fa-camera-retro"></i>

fa_icon "camera-retro", text: "Take a photo"
# => <i class="fa fa-camera-retro"></i><span class="fa-text fa-text-right">Take a photo</span>

fa_icon "chevron-right", text: "Get started", right: true
# => <span class="fa-text fa-text-left">Get started</span><i class="fa fa-chevron-right"></i>

fa_icon "quote-left 4x", class: "text-muted pull-left"
# => <i class="fa fa-quote-left fa-4x text-muted pull-left"></i>

content_tag(:li, fa_icon("check li", text: "Bulleted list item"))
# => <li><i class="fa fa-check fa-li"></i><span class="fa-text fa-text-right">Bulleted list item</span></li>
```

```ruby
fa_stacked_icon "twitter", base: "square-o"
# => <span class="fa-stack">
# =>   <i class="fa fa-square-o fa-stack-2x"></i>
# =>   <i class="fa fa-twitter fa-stack-1x"></i>
# => </span>

fa_stacked_icon "terminal inverse", base: "square", class: "pull-right", text: "Hi!"
# => <span class="fa-stack pull-right">
# =>   <i class="fa fa-square fa-stack-2x"></i>
# =>   <i class="fa fa-terminal fa-inverse fa-stack-1x"></i>
# => </span><span class="fa-text fa-text-right">Hi!</span>
```

Now we can target the text for styling:

```sass
.fa-text {
  &-right {
    margin-left: 1em;
  }

  &-left {
    margin-right: 1em;
  }
}

.fa-chain + .fa-text {
  text-transform: uppercase;
}
```