Allow creating a fat jar from sources

## Background
Currently, if you want to use spark-cobol on-premises without internet connection you need to create a fat jar yourself, from example projects for instance.

## Feature
Add sbt assembly plugin so spark-cobol fat jar could be easily created.

**`cobol-parser` and `spark-cobol` should still be published as thin jars.**

**Don't forget to update README on that.**
