Add support for tgz (tar+gzip) output
Don't include target/ directory element on output tar
New :output-dir option. Defaults to project's `target` dir.

Note: this is a breaking change, since the old default was the project's root dir.
Generating the archive under `target` directory makes more sense and plays nicer with
others like `lein clean` and `lein-package`
Release 3.0.0.