generator: Add better optionparsing

[NobbZ]

The new optionparsing should at least have the following flags/commands:

• [ ] help: explains how to use the script, also optional is to extend it with indepth documentation of each other command.
• [ ] check: checks all/given exercises if they would generate files that are different than the current ones
• [ ] diff: (optional) print a unified diff for all/given exercises
• [X] (#209) generate: (re)-generate all/given exercises
• [X] (#209) --path: specifies the path to the git base folder
• [X] (#209) --spec-path: Path to the spcification repository. Usually derived from erlang repository path. Defaults to ${path}/canonical_data/exercises
• [X] (#209) --out-path: Path to write generated files to. Usually derived from erlang repository path. ${path}/exercises

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.