--only option to extract files by extension

[PreciadoCaesar]

Hello! 👋

This pull request adds a small but useful feature to rpatool: the ability to extract only specific types of files by using the new --only option. For example:

python rpatool.py -x archive.rpa --only .png

This makes it easier to work with large .rpa archives when you're only interested in certain file types like .png, .rpyc, .ogg, etc.

Greetings from Peru 🇵🇪 and thanks for maintaining this project!

**– Cesar**

[shizmob]

Hi, thanks for using rpatool and for your contribution! I'm thinking of two things here to make it even better:

• Maybe it makes sense to follow the convention of other command-line extraction tools like unzip and specify the files as positional arguments instead?
• It would be cool to allow wildcards too and use fnmatch.fnmatch() for matching. Do you agree and if so, would you like to have a go at it? Else, I can also implement it instead.