Links shell completion files on install

Includes `shell-completion` and will run, on `postinstall`, `link bash completion/bash` and `link zsh completion/zsh.

This will, if possible, link both the bash and zsh completion files to their expected locations.

Could use some feedback on the linking of zshell's completion, as I don't use that personally.

##edit:
Removed zsh linking for now, best looked at someone actually using it.
npm adds bins to the path, so you can just do:

```sh
link bash completion/bash
```