Auto size image tag

This pull request adds feature to insert width and height attributes on generated `img` tags.

There are two ways the size attributes are added.

1. If the special `[autosize]` attribute is passed to the image renderer.
    ``` html
{% image noise.png alt="noisy" [autosize] %}
<!-- generates -->
<img src="noise-e4b10fba1cb104884016efd8e92810ab.png" alt="noisy" width="100" height="100">
```

2. If `autosize_images` is set to `true` in the config then the size attributes will be added for all images except those with width or height is already set.


Closes #114