Add support for custom formatters

Previous PRs #215 #257 #309

Currently blocked on some work in progress that will change the formatters to look like the following

// suite - an event emitter
// log - function to call with output
// options - formatter options coming from cucumber
//   snippetGenerator - function to call to generate snippet for an undefined step
//   useColors - boolean of whether or not to use colors
function MyFormatter(suite, log, options) {
  suite.on('beforeFeatures', function () {});
  suite.on('beforeFeature', function (feature) {});
  suite.on('beforeScenario', function (scenario) {});
  suite.on('beforeStep', function (step) {});
  suite.on('stepResult', function (stepResult) {});
  suite.on('afterStep', function (step) {});
  suite.on('scenarioResult', function (scenarioResult) {});
  suite.on('afterScenario', function (scenario) {});
  suite.on('afterFeature', function (feature) {});
  suite.on('featuresResult', function (featuresResult) {});
  suite.on('afterFeatures', function () {});
}

[charlierudolph]

Thank you guys very much for the offer to contribute! Just to give you some background on the work in progress. So one big thing for changing the formatters to this syntax is a complete rework of how cucumber hooks in the formatters. That resulted in the need to remove registerHandler, something that I believe is largely used for beforeAll / afterAll hooks but with an inconsistent interface. I have a branch that removes registerHandler. That however is dependent on updating to using gherkin4 which is waiting on a new release of gherkin in order to properly support the browser version.

I think I’ll just manually fix the bug with gherkin for browser support so we can keep moving. Sorry this has taken so long. I really want to get all my existing work in which should help us get close to this.

For anyone willing to contribute, please either pick up an issue or ping me on gitter with something you would like to work on. I’d ask that you create your first PR from a fork. Once I’ve worked with you to get one of your PRs in, I’m happy to give you commit access to the repo.

[benhoIIand]

Just been reading through the previous PRs and this feels like the more ideal solution - let me know if there’s anything I can do to assist in getting this pushed out 👍