Added an exclude : [path/, ...] feature.

Grunt-tree doesn't support "Building the files object dynamically".
So in return excluding files was not available.

To exclude any directory or path, simply add an array of URIs:
options : {
exclude : ['css/style.css',
'img/',
'js/bootstrap/secret.js' ]
}

Note: All paths are relative to the "current directory" / "cwd"