made glyphicons more accessible

This change will make it so glyphicons are more accessible, especially to screen reader users. The glyphicon needs to be formatted as follows:
```
<span class="glyphicon glyphicon-close" data-alt="close"></span>
```
The script I added takes the contents of the data-alt attribute and presents it as alternative content for screen reader users. It will be rendered as

```
<span class="glyphicon glyphicon-search" data-alt="search" aria-hidden="true"></span>
<span class="sr-only">search</span>
```
This does a couple of things.
* makes it so screen readers  don't try to pronounce the pseudo-element by applying aria-hidden to the element
* hides the alternative text off screen so only screen reader users hear it

Additionally, the script checks to see if the ncsu-icons font is actually loaded. It might not load because a user has overridden the fonts with their own style sheet. In this case it changes the rendered output to hide the pseudo-element off screen and show the text alternative on screen as follows.
```
<span class="glyphicon glyphicon-search sr-only" data-alt="search" aria-hidden="true"></span>
<span>search</span>
```
The way the grunt task builds the documentation, it is impossible to build the components page with the data-alt attribute in it. We would have to hand code that page or make major modifications to how the glyphicons are populated on the components page.