`mdbook init` should create a stub book.toml based on args

[budziq]

This is connected with discussion in https://github.com/azerupi/mdBook/issues/313

I currently find 3 problems in this area:

• I guess that book init should create book.toml filled with defaults (possibly from gitconfiig) and updated with info from flags such as --theme
• Currently options such as mdbook init --theme do nothing if book.toml is not present.
  Namely the book init tries to read the book.toml and does not find it, then copy_theme starts and has nothing to do as the themedir is not set. This will be confusing to the user.
• --theme option always triggers confirmation prompt (unless --force)? There seams to be no reason to prompt if the target theme directory does not exists yet.

[maccoda]

Hi,
I was wanting to have a go at this one. A few quick questions before I go down the wrong path.

1. create book.toml filled with defaults

   In regards to this should it be the entire set of defaults or only a subset? The main reason I am asking is that elements like the title, author, and description are all optional, hence writing these in there will just result in empty elements. Of course following your point of looking at the gitconfig we should be able to at least populate the author but for the other two I am not too sure how we should handle that.
   Edit: Sorry I hadn't run the init command in a while and have just seen that the book.toml is created and does already contain a subset so please disregard this part. I will see what I can do about the theme and author information.

2. In regards to using the gitconfig, what would be your preferred method? I looked at how cargo handles it and it uses the git2 crate I believe. Did we want to follow this example or is that a bit of overhead considering we will be wanting one line perhaps? Also whilst looking at cargo I saw that they will support several other vcs (most of which I am not too familiar with...) did we want to support those if gitconfig was not found?

I will make a start on this just using file reading for the gitconfig and publishing the full set of defaults for the configuration for now but let me know if you would like to change it up 😄