[Enhancement] Add option to exclude .gitignore-ed files

As a developer and user of `shhgit` scanning local directories I most likely will have local files like for example `.env` files containing secrets which are protected through the full-disk-encryption of my machine and prevented to be accidentally committed through the `.gitignore` or `~/.gitignore_global` file.

Expected behavior:

- I can specify an option to exclude those files matching gitignore-entries from the search

Actual behavior:

- I have to filter those entries from the `shhgit` log after the scan finished

Though this option should **not** be enabled by default as it reduces the visibility of secrets lingering around on the disk (which shouldn't exist in the first place) which could be committed through `git add -f`…