feat: add support for -I

[ByteBaker]

• also add test case
• remove fn remove_empty_directories_from_cache as tree command displays empty directories
• doing the above fixed another test case
• change type of CLI parameter for -P from Option<String> to Vec<String> to be in accordance with tree command
• fix expected result test test_filter_txt_files

TODO:
Test test_filter_txt_files still breaks. The expected value is based on tree's output, therefore it's correct. The logic in this crate needs to be validated and corrected.

[ByteBaker]

@sighol This one is not final by any means. I need your help/advice here.

I took liberty in deleting fn remove_empty_directories_from_cache because tree does not exclude empty directories and this was causing a test to fail where the folder tests/simple/yyy/k was being omitted in the output.

One of the tests still breaks. The expected value is correct and based on tree's output. I need to check why the other directories aren't being listed after a match is found for the glob when running -P "*.txt".

Your advice would be much helpful. You can merge this one should you deem fit, but I'd personally prefer to fix the test before we do so.

[ByteBaker]

I ended up debugging this and turns out we had a lot of redundant code convoluting the logic inside FilteredIterator. We didn't need the fields cache, skip, or next_item.

• skip and cache were redundant because a cache is already present inside FileIterator. We were just taking the files from FileIterator::cache and putting it inside FilteredIterator::cache (and sometimes inside next_item too). All of this is gone now.
• skip was a flag determined in the beginning if any filters were present or not. These are being handled inside FileIterator and the flag made no sense so I removed it too. As a result fn FilteredIterator::skip_filter is also gone.

All test cases now pass, output always matches that of tree command. Lemme know what you think. If everything looks good, please go ahead and merge it.