Added extends plugin

```
function git --description 'Extends git command'
    _extends $argv
end

function _git-hi --description 'Shows version and hi'
    set -l git_version (command git --version)
    echo "Hi from $git_version! Feeling adventurous? Try `git help` ."
end
```

and then you just call it:
```
┌─<josemi @ WW-MBP-Josemi in ~>
└─<>──» git hi                                                      0 < 18:03:21
Hi from git version 1.9.3! Feeling adventurous? Try `git help` .
```