Is partials option relative to target template or root dir
Hi, I saw there is a `partials` option which can set handlebars partial.
I expect it to be relative to the root dir that I passed to `views()`, but it seems it's relative to the target template?

For example, I have the configuration like this:

```js
const app = new Koa()
  .use(views('lib/views', {
    map: { hbs: 'handlebars' },
    options: {
      partials: {
        head: './partials/head'
      }
    }
  }));
```

`lib/views/partials/head.hbs`:

```hbs
<head>
  <title>Hello World</title>
</head>
```

`lib/views/index.hbs`:

```hbs
<!DOCTYPE html>
<html>
  {{> head }}
  <body>
    <h1>Index</h1>
  </body>
</html>
```

`lib/views/pages/new.hbs`:

```hbs
<!DOCTYPE html>
<html>
  {{> head }}
  <body>
    <h1>New</h1>
  </body>
</html>
```

It works fine with `lib/views/index.hbs`, but got error from `lib/views/pages/new.hbs`:

```bash
Error: ENOENT: no such file or directory, open '...\lib\views\pages\partials\head.hbs'
```