Request: Add support for text files

[Kigstn]

Hi! In my workflow, we have a lot of configs that are in .yaml / .toml files. Also, we have a ton of readme.md files.
Currently, these get ignored as shippie does not whitelist them.
https://github.com/mattzcarey/shippie/blob/main/src/review/constants.ts

Can we add those pure text files to the whitelist - or better yet, have an input arg where you can set allowed file ending? :)

Cheers & thx for the cool tool!

[mattzcarey]

sure. feel free to open a PR to add them :)