Absolute path for options

Hi, I would like to be able to provide an absolute path for the external data passed to pages,

for example:

```
panini({
root: 'src/pages',
layouts: 'src/layouts',
partials: 'src/partials',
helpers: 'src/helpers',
data: '/tmp/user123/'
})
```

Currently it appears that all option paths has to be in the current working directory of the (gulp) process