Here's something simple that worked for me.
- Extract vertices from your line layer.
- Install Shape Tools Plugin. Use Create Shape function and create a "line of bearing". For the Azimuth Field input select ANGLE. For the distance field, nothing. Then specify a standard length for all the lines.
- Run it and you'll get a new line layer with the extended lines. If you need the lines to go the other way, reverse the azimuth values.
