Add --environment to supervizer command

This merge request is a response for Issue #5 

You can add environment variable to application simply with --environment variable (or -z) : 

```bash
supervizer add -n api -s /home/xxxx/server.js -z NODE_ENV=local -p 3000
```

You can add multiple Environment variables with `,` separator : 

```bash
supervizer add -n api -s /home/xxxx/server.js -z NODE_ENV=local,FOO_ENV=foo -p 3000
```