Removing no-longer necessary file.expandDirs and file.expandFiles methods.

* file.expandDirs(…) -> file.expand({filter: 'isDirectory'), …)
* file.expandFiles(…) -> file.expand({filter: 'isFile'), …)
The file.expand 'filter' option can now be either a string or a function.

* The string has to be a valid a fs.Stats method name.
* The function is synchronous, accepts a relative filepath, and should return true or false.