Add category_id_start parameter to support starting category IDs from 1

[xCatalitY]

This PR introduces a new parameter category_id_start to allow users to specify the starting value for category IDs when converting labelme annotations to COCO format. By default, the category IDs start from 0, but users can now set category_id_start to 1 or any other desired starting value.

Changes:

• Added category_id_start parameter to get_coco_from_labelme_folder function in labelme2coco.py
• Updated convert function in __init__.py to accept category_id_start parameter and pass it to get_coco_from_labelme_folder
• Modified app function in cli.py to include category_id_start parameter
• Updated README.md to document the usage of category_id_start parameter in both basic and advanced usage sections

Additionally, this PR fixes an issue where the file paths were incorrectly formatted in the "file_name" attribute when data is structured in subdirectories. Previously, if an image was located in a subfolder, e.g., subfolder_1/image.jpeg, the "file_name" attribute would only contain image.jpeg instead of the complete path. This has been fixed to include the full path in the "file_name" attribute.
This resolves #17

[xCatalitY]

Did your requested changes

[fcakyon]

@xCatalitY Thanks a lot for your contribution!