Provide easier access to commitizen config to prompters

Currently when writing a prompter, e.g., cz-conventional-commit, if you wish to integrate with commitizen configuration then you need to do the following:

```
var configLoader = require('commitizen').configLoader;
configLoader.load() // synchronous, not a promise
```

I think it'd be nicer to actually have commitizen inject said configuration into prompters, e.g.,

```
module.exports = {
prompter: function myAdapter(inquirer, config, commit) {
}
}
```

This would remove the backwards dependency from prompters to commitizen, and reduce the need to test that config actually was loaded correctly when testing prompters, and allow for standardised configuration between all prompters. It could also be done in a backwards compatible manner by checking the length of the prompter function (that tells you how many arguments it expects)