Add --output flag for custom output file

[calchiwo]

Add a flag to allow writing the generated explanation to a user-defined file instead of always writing to EXPLAIN.md

Example:

explainthisrepo owner/repo --output preferred-file-name.md

Current behavior always writes:
EXPLAIN.md

With this change, users could specify a custom output file.

Implementation idea:

• Add a --output flag to the CLI
• If provided, write the generated explanation to the specified file
• If not provided, keep the default behavior EXPLAIN.md

This should work:

explainthisrepo owner/repo --output notes.md

explainthisrepo owner/repo --output path/to/directory/file-name.md

explainthisrepo owner/repo --detailed --output file-name.md

This would make it easier to generate multiple explanations without overwriting the default file.