More progress on BashCompletionGenerator

- Avoids generating unecessary script segments
- Honours completionBehaviour and completionCommand on @Arguments
  annotations
- Various indentation and formatting fixes
Get Bash completion working properly

- Fixes some syntax errors in generated scripts
- Fixes issues with returning completions from nested function calls
- Adds support for generating scripts with debugging enabled i.e. set -o
  xtrace
Preparation for 0.8 release

Update README, add CHANGELOG