Add case-insensitive equality check

I would really prefer something like

```
expect(something).to.equalCaseInsensitive(other);
```

over

```
expect(something.toLowerCase()).to.equal(other.toLowerCase());
```

Can we have this?

Start by reviewing the existing equality assertion API and its tests to find where a case-insensitive matcher would fit. Done means supporting the requested equalCaseInsensitive syntax and covering comparisons with the existing test suite.

Written by the indexing model from the issue text.