add extra instructions option

[MarkArts]

This adds an option EXTRA_INSTRUCTIONS which will be added to the prompt. This can be used to fine tune reviews based on the repository.

For example by default, on adding the github action to the repo trough a PR, it will comment on Please add a newline at the end of the file to follow the best practices for POSIX standards.. Which is something that can be better enforced by linters. This change allows you to add intructions to ignore comments on linting and formating