properly support compact and full grunt task syntax

The usual way in grunt multitask to define source and destination files are <tt>src</tt> and <tt>dest</tt> properties (https://github.com/gruntjs/grunt/blob/0.3-stable/docs/api.md#thisfile--grunttaskcurrentfile)

In this pull request I have added support for it, while keeping backward compatibility.

This way it is possible to generate bundle which filename contains e.g. package version, or use compact task format.