Size isn't everything

This release changes:

• The browser window is no longer resized to fit the contents before taking a screenshot by default. This caused issues with responsive CSS which could trip a breakpoint and make the display render differently to that expected. This behaviour can be re-enabled by setting the :resize-to-contents? option to true.

This release adds support for:

• The :assert? option (enabled by default) allows you to decide whether to assert that the actual image matches the expected image - good for when you just want to capture a screenshot without asserting, or to disable a test