Refactoring `book:build`

[bagasme]

I'm proposing to refactor book:build task on Rakefile.

The task will be split into:

1. generating book/contributors.txt (as prerequisites)
2. one build task for each supported formats (html, epub, and pdf)

Also introduce book:clean task, which will remove all generated artifacts (book/contributors.txt, progit.{html,epub,pdf})

[jnavila]

While you are at it, I would also like to split the tasks of checking some artifacts (html, epub), from the task of building all of them (pdf). PDF takes a lot of time and does not check anything, so it is useless for CI.

[bagasme]

@jnavila ok, but should book:build still check artifacts (html and epub, minus pdf), right?

[jnavila]

We could have a target book:ci which only builds for checking and book:build which only focuses on artifacts. The check could be performed for book:build but errors would not lead to failure.

[bagasme]

@jnavila did you mean book:ci only for checking the generated books? And should book:ci be default task for CI?