Inlining of each asset type should be optional

I find inlining of scripts and styles to be extremely useful, but sometimes don't want to inline my images (especially in image-heavy content where there's a bigger trade-off between time to initial render and number of network requests).

By adding the ability to disable inlining for each asset, this module becomes more useful to people who would like to use some of its power, but not all of it.

There are no changes to the current behaviour, other than a few boolean flags, and I've updated the tests to ensure each of the options works as expected, and that when you add all of them, the out-stream matches the in-stream.