Add built-in First and Last item functions

Sometimes it's useful to take just the first item on a list, or the last. This is currently very cumbersome, to the point that it's "easier" just [write an inline task](http://blog.seravy.com/msbuild-tasks-first-item-in-an-item-group/) to do so.

A new `First()` and `Last()` item function would simplify things a lot.

Usage example:

```xml

```

Renders:
```
v1.3.0
release
```