Add abstractions for generation of Html string

[IsaacWalker]

The primary module exported from the core library, and consumed by the testing library integrations, is an interface that takes in the results from an axe run and returns a Html string.

In the future it may need to support:

1. Handle the input of images in some form (e.g. datauri).
2. Instructions on what rules/groups to display.
3. Other theming and layout parameters