Add MacPort's default path to search PATH

[saagarjha]

The default highlight installed by MacPorts at /opt/local/bin didn't seem to be picked up by QLColorCode, so I added it to PATH.

[saagarjha]

FYI: you might be wondering why this is necessary; it seems to work for an obscure user defaults bug where the highlight path is somehow not set correct and QLColorCode performs the lookup and fails, but doesn't add /opt/local/bin/highlight to preferences correctly. It's your choice whether you think this is a big enough issue to merge in.

[anthonygelibert]

Thanks for the job!