Auto completion for local files

It would be handy, if s5cmd could support auto-completion for local files.

For example
```
> ls -l
total 0
-rw-r--r--  1 ilkinulas  staff  0 Aug  7 23:22 a.txt
-rw-r--r--  1 ilkinulas  staff  0 Aug  7 23:22 b.txt

> s5cmd cp <TAB>
```
should allow me to select `a.txt` or `b.txt`.