Add symlinks metadata API

Note that the symlinks have to be created after the regular files are written. This is because creating a symlink to a file that doesn't exist may fail.

A result of this is that nested symlinks (foo -> bar -> qux) are a little trickier to implement and have been left unsupported. Circular symlinks probably shouldn't ever be supported.

I'll be working on that symlinks feature for [metalsmith-permalinks](http://github.com/segmentio/metalsmith-permalinks) tomorrow, so we'll get to test this new API with an actual feature.