Support for projects in organizations

[Seldom-SE]

I renamed repo_owner to username, so the terminology would fit these changes. This is a breaking change!

Also, I made it so that the tool looks for an organization if repo_name is left empty. Unfortunately, users may have to read the docs to understand this. Feel free to make suggestions or contributions for this.

I have updated the docs to reflect these changes.

Closes #14

[thehale]

There might also be a way to directly look up the project without going through the organization/user.

This could also allow us to use the REST API instead of the GraphQL API which would mean that public repos wouldn't require a personal access token to generate a bit down chart.

Relevant API documentation:
https://docs.github.com/en/rest/reference/projects#get-a-project

[Seldom-SE]

It looks like each project has a unique ID associated with it, which is what is used in the API. However, as far as I could tell, there is no way for a user to find this ID on GitHub. So, even if we used the REST API, the user would still have to deal with organizations/users. I'm not sure what that would mean for the personal access token, though.