Replacing message formats to be ${stringKey} rather than ${numberIndex} based
Do not pre-format customMessage - do it during the final interpolation
Refactoring exception assertion to go via defineAssertion(), so that we can have one less method for fail()ing (i.e. this.fail() or referee.fail() should be used)
Merge pull request #11 from dominykas/buster-issue-274

Replacing array index based formats with object key based ones