Chapters 16 & 17: templates and new formats

[dr-harper]

Chapters complete (I hope). I have looked through your edits to my previous PRs and tried to reduce the number of changes you will need to make, but I am sure there are still some stylistic issues you will want to change.

I really wanted to write a lot more about creating custom pandoc templates, but feel this is a complex topic and requires an understanding of LaTeX and HTML. So I have included a single example to at least show how we can use pandoc variables in a template. This is a subject I would like to expand in the "Mastering R Markdown" book though, as I think it is really powerful.

I have also made a few example templates and contained them within a repository: https://github.com/mikey-harper/example-rmd-templates . I wasn't sure whether you wanted me to keep them as part of the book, but if you do like the idea I will expand the description and include a README.md in the repository.

Let me know if there are other things I can help out with on the book for now :) I was going to read through it all and pick out any typos and see if I had any other suggestions.

[yihui]

Thanks! I'll take it from here, and you can start thinking about "Mastering R Markdown" now :) I'll let you know if I need further help with this book.

[dr-harper]

Thanks for the tweaks, and also keeping the reference to my templates. I will update the repository a bit to include a README etc.

Started working on the book. I am sure I will call on your for some feedback for ideas soon!