Added extends plugin

I see. So,
```fish
function ls
  _extends
end
```
Would simply wrap `ls` and pass the arguments along, no harm done.

Can you provide an example of how it can be used to extend `ls` in an useful way?

For example, how does it compare to

```fish
function ll --description "List contents of directory using long format"
	ls -la $argv
end
```