feat: Add support for ignoring standalone tasks and role tasks in the graph view

[Eltryo]

Premise

When running the ansible-playbook-grapher on a huge project folder the resulting svg or mermaid graph happens to be quiet confusing with all the task nodes being rendered. So far there was no option to view all roles (also nested roles) without viewing the corresponding tasks, so I implemented a feature to do exactly that.

Changes

• Add entry for the new option --only-roles in README.md
• Add the option in cli.py and adjust grapher.py and parser.py accordingly
• Update docstrings accordingly
• Add logic to prevent tasks from being added
• Lint project with the corresponding make target
• Add cli and parser tests

Screenshots

without --only-roles

with --only-roles

[haidaraM]

Hi, thanks for your contribution. I just approved the workflows, and I'm rebasing your branch to see if the checks pass.

[haidaraM]

Hmm, I have to make a few changes to the workflows so that they work with forks. Will keep you in touch.

[Eltryo]

Hmm, I have to make a few changes to the workflows so that they work with forks. Will keep you in touch.

Ah alright, thank you!

[coveralls]

coverage: 94.298% (+0.02%) from 94.274%
when pulling 5db9d9d on Eltryo:feature-ignore-tasks
into 0e7485c on haidaraM:main.

[haidaraM]

Small changes to make.

Other than that, this seems to be a reasonable feature to add