[WIP] Support auto-completion.

Update to @OriPekelman's #87 

With this, you can add:
```
platform _completion --generate-hook | source /dev/stdin
```
to your `.bash_profile`, `.bashrc` or `.zshrc`

For ZSH you might need to change `--generate-hook` to `generate-hook=zsh`.

No custom completion yet, but this should be easy at least for a list of projects or environments.