Rename it_behaves_like_a to include_context.

This introduces fewer word concepts, and solves the awkward a/an
problem. Not putting in public API yet because behaviour is unexpected
with nested contexts and `let`.
Extract common EvaluateFailed exception.
Cute shorthand for XSpec::Evaluator.
Add -e option to xspec runner.
Fix errant stdout pollution.
WIP
Fix include_context rename
Documentation fixes.
Rename opts to config.
Doc for short ids.
More useful flag combining for runner.
Merge branch 'runner-wip'