Add options to ignore directories

Currently all directories are walked through, except the ones ignored by `.gitignore`

it would be nice to be able to ignore the walkthrough of some directories, maybe because they are big and do not contain python code.

or in my case if they crash the walkthrough. I'm using pdm and the `.venv/` at the root of my project contains symlink to python versions outside of the project and it raises an error:

```py
ValueError: '/home/eugene/.pyenv/versions/3.10.5/bin/python3.10' is not in the subpath of '/home/eugene/my_projects' OR one path is relative and the other is absolute.
```

for now I'm resorting to:

```
fixit fix -a $(ls -A -I.venv)
```

Start with the `fixit fix` command and its directory-walking behavior, especially how `-a` receives paths and how `.gitignore` exclusions are applied. Define and test an explicit way to exclude directories so paths such as `.venv/` are not walked and the reported symlink error is avoided.

Written by the indexing model from the issue text.