Add package 'xml2'.

Provides some tools for manipulating XML, HTML, and CSV in shell pipes. For example using the `xml2` tool we have

```
$ nix-env -f . -qa xml2 --xml --meta | xml2 | grep -A2 /items/item/meta/@name=homepage
/items/item/meta/@name=homepage
/items/item/meta/@type=string
/items/item/meta/@value=http://ofb.net/~egnor/xml2/
```