Added option to customize the output path of the manifest file

This PR makes it possible to customize the output path and name of the manifest file. It allow fixes a bug where the build would fail if /assets/ directory did not exist in the destination.

- Added manifestDest option which can be specified in Brocfile
- manifestDest option takes a function
- manifestDest function is bound to the instance of the Fingerprint Filter
- manifestDest receive 2 arguments: destDir and hash of the content
- manifest file is written to the file system with [fs.extra.outputFile](https://github.com/jprichardson/node-fs-extra) which automatically creates parent directories if they don't exist
- added fs-extra as a dependency in package.json