Highlighting of authors (and other parts of an entry)

This commit allows to specify a pattern which is then highlighted in an entry. The most common use case would be to display your own name in bold (Example: http://www.martinhenze.de/publications/). This has also been requested on wordpress.org (http://wordpress.org/support/topic/how-to-display-my-name-in-bold-1).

I simply added a new option to the short code, which allows to specify an arbitrary regular expression to match for.

**Example usage:**

Highlight each occurrence of *M. Mouse*:
```
[bibtex highlight="M. Mouse"]
```

Highlight each occurrence of *M. Mouse* or *D. Duck*:
```
[bibtex highlight="M. Mouse|D. Duck"]
```