Add option to append a folder to the default directory as a base dir

[Twigonometry]

If default directory has been overriden, have option to use it as a base directory and append a suffix to the source

For example:

• default_dir="/home/user/vault/writeups" in .env
• run with command line argument to use default dir: python3 writeup-converter.py DEFAULT /path/to/attachments/ /path/to/target/ /path/to/target/attachments/ (will read from /home/user/vault/writeups/ as the source)
• run with a flag to append a string to the base dir: python3 writeup-converter.py --base-append "/htb/blue" DEFAULT /path/to/attachments/ /path/to/target/ /path/to/target/attachments/ (will read from /home/user/vault/writeups/htb/blue/ as the source)