use global config to parse template variables in basePath strings

When overriding a basePath in a template, this allows for variagles from `grunt.config.get` to be used. Example

```javascript
grunt.initConfig({
	buildDir: 'build/',
	includeSource: {
		options: {
			basePath: '<%= buildDir %>/js/app'
			baseUrl: 'js/app/'
		},
		all: {
			'<%= buildDir %>/index/html': ['<%= buildDir %>/index/html']
		}
	}
})
```

```html
<!-- include: "type": "css", "basePath": "<%= buildDir %>/css", "baseUrl": "css/", "files": "**/*.css" -->
<!-- include: "type": "js", "basePath": "<%= buildDir %>/js/vendor", "baseUrl": "js/vendor/", "files": "**/*.js" -->
<!-- include: "type": "js", "files": "**/*.js" -->

```

In this case, all of the `<%= buildDir %>` tags become `'build/'` before looking for files.