Tags.md not always processed last

I found that linking to tags.md will make it not be processed last. This made tags.html content incomplete after a gitbook build.

```
# Table of Contents

* [item1](/tags.md#item1)
* [item2](/item2.md)
* [tags](tags.md)
```

In the above example, any tags in item2 will not show after a gitbook build.

There is a workaround, but I thought I'd post it here, since it took me a while to track down.