Allow custom esprima version.

This allows usage of `harmony` branch of Esprima to provide ES6 support.

I realize that this likely shouldn't be a user-facing feature so I have not updated the bin script or CLI, but this allows usage of JSCS for projects that are already using ES6 module syntax.

For reference this simple script is what I have been testing with (in Ember source):

```javascript
var esprima = require('esprima');
var configFile = require('jscs/lib/cli-config');
var Checker = require('jscs/lib/checker');
var reporter = require('jscs/lib/reporters/console');

var checker = new Checker(false, esprima);
var config = configFile.load();

checker.registerDefaultRules();
checker.configure(config);

checker.checkPath('packages/ember-metal')
  .then(function(results) {
    var errorsCollection = [].concat.apply([], results);

    reporter(errorsCollection);
  });
```

In this case the version of Esprima is used from the including repo (instead of the stable version that is included with node-jscs).