[WIP] Adding new step definitions for scenarios that don't require classnames

For discussion, as previously mentioned in comments on #248.

Plenty of features don't need to care about the exact names or paths to the spec or SUS, they just want to talk about specific examples the spec contains.

This is a simple implementation of that sort of syntax.

I can see us having a few different levels:

1. Example scenarios that talk about single examples and don't specify the rest of the spec's contents (as I have in this PR)
2. Spec scenarios that talk about multiple examples (e.g. testing let() behaviour between examples) but which don't care what the spec/SUS are actually named. This can be achieved with something similar to the definitions I've added here
3. Spec scenarios that talk care about paths/names, e.g. the existing Code Generation scenarios that we have already.

Any comments before I go further?