Use semantic markup for highlighted phrases in search results

The search block currently produces presentational markup for highlighting the search phrase in results. For example:

```
<span style="background-color: #EFE795">highlighted phrase</span>
```
I feel it would be better to use an element with more semantic value (I've used _strong_) and encourage users to style this element in their CSS. For example:

```
<strong>highlighted phrase</strong>
```

For backwards compatibility, I've left in the setHighlightColor() method, and if the user specifies a color this way it will still be included as an inline style.