Add `@/at` format to import helpers from template

This allows you to reference helpers directly inside the handlebars template. The `@` format syntax is the same as the [require-hbs plugin from Guy Bedford](https://github.com/guybedford/hbs#helpers).

## Format: 
`{{@helpers/myhelper}}`
`{{#@helpers/myblockhelper}}...{{/helpers/myblockhelper}}`

Added path map option(`helpersAtPathMap: {}`) to map a module name (such as "helpers") to a complete path making it easier to reference helpers.

Also added documentation on this new `@` format.

---

Example taken from added documenation to readme.md:

## Example: inline `@/at` helpers

#### `src/hello.handlebars`

```handlebars
<p>Hello {{firstName}}</p>
<p>HELLO! {{@helpers/capitals.helper firstName}}</p>
{{#@helpers/noop.helper}}
	<p>A block helper that invokes the block as though no helper existed.</p>
{{/helpers/noop}}
{{> footer}}
```

#### `src/js/hbs-helpers/capitals.helper.js`

```js
module.exports = function(str) {
    return str.toUpperCase();
}
```

#### `src/js/hbs-helpers/noop.helper.js`

```js
module.exports = function(options) {
	return options.fn(this);
}
```


#### `gulpfile.js`

```js
var gulp = require('gulp');
var handlebars = require('gulp-compile-handlebars');
var rename = require('gulp-rename');

gulp.task('default', function () {
	var templateData = {
		firstName: 'Kaanon'
	},
	options = {
		ignorePartials: true,
		partials : {
			footer : '<footer>the end</footer>'
		},
		helpersAtPathMap: {
			'helpers': './src/js/hbs-helpers/'
		},
	}

	return gulp.src('src/hello.handlebars')
		.pipe(handlebars(templateData, options))
		.pipe(rename('hello.html'))
		.pipe(gulp.dest('dist'));
});
```

#### `dist/hello.html`

```html
<p>Hello Kaanon</p>
<p>HELLO! KAANON</p>
<p>A block helper that invokes the block as though no helper existed.</p>
<footer>the end</footer>
```

---

Currently, in the current master branch of this project, the only way to batch helpers is to do something like this:
```
helpers : (function() {
	var helpers = {};
	// Get a list of all the helpers in our holder
	var helperFiles = glob.sync("./src/js/hbs-helpers/*.js", {});
	helperFiles.forEach(function(filePath, index, array) {
		var pathParts = parsePath(filePath);
		try {
			helpers[pathParts.name] = require(filePath);
		} catch(e) {

		}
	});

	//console.log(helpers);
	return helpers;
})()
```