Allow a list of objects to specify commands to run

As a workaround for #5, this commit adds the ability to specify a list of jobs in 'long form' and can be mixed with the original string form.

The object has 3 attributes:

- `cmd`: The command to run.
- `args`: Arguments to the command.
- `env`: Env vars which will be merged with `process.env`.

```javascript
var jobs = [
	{
		cmd: 'ls',
		args: [ '-la' ]
	},
	{
		cmd: 'uptime'
	},
	'ls -la'
];
exec(jobs);
```