(feat): Add Append Option to CLI

[octehren]

Feature summary

Add CLI option to append to an output file rather than overwrite it.

Problem / motivation

The --include_patterns and --exclude_patterns options allow for some pretty neat CLI-fu already. However, sometimes I just want to append two files or directories to a single digest file and make it as trivial as it sounds.

Proposed solution

Add a new CLI option, --append, that allows the user to concatenate the command's output to an existing output target rather than always overwriting it.

Alternatives considered

Possible to create differently named output files & combine then and also to twist include & exclude patterns to grab different directories and/or files at once. However, this feature implementation makes this process trivial.

Which interface would this affect?

CLI

How important is this to you?

Important

Would you like to work on this feature yourself?

Yes, I'd like to implement it

Would you need support from the maintainers (if you're implementing it yourself)?

No, I can handle it solo

Additional context, screenshots, or examples

I already have a local fork of gitingest with this feature working.
Here's the PR: #564

Here's a screenshot with its functionality in action: