Improve code for toml file handling and remove code duplication from new.rs file

[Bas-Man]

I have been looking at code regarding issue #130 and which was referenced by pull request number #142.
I have some comments and ideas on how to improve this.

Issues

• File new.rs contains a lot of information regarding how Site and Issue struct data is to be written to their toml files. I think this can be removed by making struct Site{} and struct Issue{} directly responsible for writing their related data to file using the the toml crate.
• When creating a new Issue there currently does not appear to be any protection to prevent a user from intentionally overwrite an exiting Issue.

Ideas

• Have struct Site{} manage its own zine.toml file directly. Work in progress with pr Refactor how Site struct is created #151
• Have struct Issue{} manage its own zine.toml file directly. Work in progress with pr Feat: Add a way to create a new issue by chaining #150
• Have struct Article manage its own data and append it to the Issue zine.toml. This may have the added benefit of being able to remove an article programatically in the future. This is a work in progress with pr Initial commit to Support new Article features #153. It will require author.rs to have the Trait FromStr implemented based on my current thinking.

[Bas-Man]

These drafts are ready for a quick review and comments.
I have done it this way to avoid breaking any existing code. And to also make it easier to avoid merge conflicts.
Once this pull requests are complete and merged, the code in new.rs can be updated to use them. There may need to be some additional fixes during that process.
@Folyd

[Bas-Man]

I will submit a new pull request to replace the ones I have just closed.

[Bas-Man]

I have issued a new pr #162 which will close this issue #152

This should make it much easier to add new features. And also make it much easier to add the zine new --article support.

[Bas-Man]

@Folyd

Can you tell me what issues you have with my three PRs. They are all related. Each pr builds on the previous to create the desired "--article" command line option.

I can make one more attempt to get to something you can work with. But they all need to be implemented for the "--article" option to work.

[Folyd]

Hi, @Bas-Man. I apologize for the delay in my response. Thanks for working on this. I hope you can submit a single PR with as less changes as possible to support the --article. What do you think?

[Bas-Man]

Nearly all the changes are required. Each change builds on previous to make things clean and maintainable moving forward.