Pack: new pathFilter parameter

This pathFilter parameter is necessary to create package for Debian or OpenEmbedded:
the tar files inside dpkg or ipkg package must have path like "./usr/bin/foo",
i.e. with a leading './'.

pathFilter expects a function which is run on each file or directory stored
in the archive. This takes path as a parameter and is expected to return
a modified path. For instance:

```javascript
tar.Pack({
		noProprietary: true,
		pathFilter: function(p) {return './' + p ;}
	})
```