Polish Book.txt workflow

[cansavvy]

Summary

Previously there was a lot going on that was unclear in the bookdown_to_book_txt() function and when I ran the main leanbuild::bookdown_to_leanpub() function it kept erasing my Book.txt file that I wanted.

So a few changes to make this more user friendly:

• make_book_txt argument is now set to default FALSE so you don't accidentally create a Book.txt file you don't want.
• if make_book_txt is FALSE it will look for your Book.txt file and copy it over to manuscript folder
• if make_book_txt is TRUE, it will read in the md files in the quiz directory, and based on the numbers in the titles, will make a Book.txt file where the quizzes follow the chapters of the same number.