Account for suburi in img helper

Currently, the `img` helper does not prepend the suburi to the generated image tag output (see #136). For example,

```
!= img '/images/foo.png'
```

will output

```
<img src='/images/foo.<cache-buster>.png' />
```

This is a problem when the app is deployed to a suburi in production. My thought is the `img` helper should behave similarly to the `js` and `css` helper by prepending the suburi to the generated html output.