Support assertions with two arguments.

Going to support assertions with two arguments.

Support these methods by default,

- `equal`
- `notEqual`
- `strictEqual`
- `notStrictEqual`
- `deepEqual`
- `notDeepEqual`

and provide options to add more methods to instrument.

TODO
----------
- [x] Tests for two args case
- [x] Make method names configurable
- [x] Refactorings