Add a task to purge from Fastly

This can be used as:
```
fab production cdn.fastly_purge:'/one,/two,/three'
```

It could also be used following a cache purge:
```
fab production cache.purge:'/one' cdn.fastly_purge:'/one'
```

This [Fixes #69166246]
Merge pull request #48 from alphagov/cdn-purge

Add a task to purge from Fastly