Implemented crop closeness

The current approach of doing cropping is similar to that of Thumbor where each image has a "focal point" which Wagtail would crop to.

This isn't good when displaying large images with small focal points on your homepage for example where you don't want much cropping.

This pull request adds a new optional parameter to ``fill`` which allows the template author to choose how much cropping/resizing should happen: ``-c<percentage>`` (eg. ``fill100x100-c0``, ``fill100x100-c50``, ``fill100x100-c100``).

Examples:
 - Rendering a thumbnail ``fill-100x100-c100``. This will make a small image that is cropped closely to the focal point (similar to Thumbor).
 - Rending full size (eg, on the homepage) ``fill-500x500`` (defaults to ``-c0``). This will only crop the image to change the aspect ratio. The focal point is still used to determine which side of the image to crop from the most (Similar to Squarespace).
 - Something in between ``fill-300x300-c50``. This will do some cropping and some resizing.

TODO:
 - [x] Add onto URL generator
 - [x] Tests
 - [x] Docs