Cleanup tests also;

Consider a width or height set on an arbitrary position on the
terminal. The content will cause wrapping, which is most probably not
what the user intended.
Fix colour output in JSON.
Update simplecov gem version to 0.9.0.
Replace StyleCollection with Style class;

This allows the retention of the original value provided by the user which is useful when returning JSON, as we shouldn't return the escape sequence.
Remove Oj gem dependency, and use JSON from the standard library.
Tidy up tests.
Remove #as_hash from Colour as no longer needed.
Add improved test for Output.render.
Add or reduce sizes of created interfaces.
Tidy up tests.
Rather than skipped tests, just mark them with :nocov:.
Replace Geometry with Coordinate class, also;

- Add 'centred' attribute for Interface.
- Update interface names in tests for debugging.
- Tidy up tests.
- Add 'pry' back into test_helper.