Add symlinks support

As I implemented this, I realize that symlinks still leave a lot to be desired when it comes to linking. Nevertheless, it may be useful for builds that have large asset directories. The main benefits as I see it are reduced build times and disk space.

I haven't been able to test on Windows, but I'm not sure that's a priority. This works fine when I test it with [my website](http://github.com/srcreigh/srcreigh.github.io) and I've also added a test with this commit.