update nimibook to 0.3 + much needed maintenance WIP

[pietroppeter]

apart from fix needed to update to Nimib 0.3, this PR does a much needed maintenance for nimibook which includes also taking advantage of stuff introduced in Nimib 0.2...

• [x] fix blocks in document.mustache
• [x] change templates folder to srcDir / templates
• [x] refactor code
  • [x] rename docs to themes
  • [x] rename books to commands
  • [x] rename publishers to builds
  • [x] refactor assets code from tocs to its own module
• [x] add readme in src to explain code structure
  • [x] add link to it from main README.md
• [x] remove populateAssets from newBookFromToc
• [x] load document mustache from memory (fixes https://github.com/pietroppeter/nimibook/issues/32)
  • [x] add in theme (aka docs maybe rename?)
  • [x] remove management of templates in assets
  • [x] remove templates from code in examples/ and in book/
• [x] more use of config (both nimib and nimibook)
  • [x] allow Book fields to be set in config
  • [x] add nimib cfg field in Book object
  • [x] load nimib and nimibook config while creating a book in newBookFromToc
  • [x] refactor newBookFromToc into initBookFromToc
    • [x] extract tocDsl
  • [x] take advantage of srcDir/homeDir in newBookFromToc and everywhere I can (e.g. assets, commands...)
  • [x] use config for nimibook
• [x] fix https://github.com/pietroppeter/nimibook/issues/46 (make title same as mdbook default: chapter name - book title)
• [x] change book.json target folder to docs (a better place) (fixes https://github.com/pietroppeter/nimibook/issues/23)
  • [x] fix in theme.useNimibook
  • [x] fix in commands
  • [x] do not removed book.json at the end of build
• [x] adjust gitignore
• [x] I should also fix this: https://github.com/pietroppeter/nimibook/issues/44 (rename draft entry)
  • [x] fix also rendering of draft
• [x] added book.renderToc
• [x] improved check: fails after checking both sources and output and reports failures
• [x] refactor clean (use Src and Out as names instead of Root and Doc)
• [x] add srcPath outPath in entries
• [x] improve init
  • [x] empty markdown should contain title
  • [x] empty nim should contain minimal working file (init, text with title, save)
• [x] bump to 0.3
• [x] testing implementation
  • [x] help (no args)
  • [x] init
  • [x] dump
  • [x] check
  • [x] build
  • [x] clean
  • [x] update
• [x] fix some broken rendering (theme selector does not work) <- issue with defaults overriden by toml
• [x] build should fail only at the end reporting errors
• [x] remove default parameters and allow to pass parameters to build, see debug builds: https://github.com/pietroppeter/nimibook/issues/49
  • [x] remove all default parameters
  • [x] allow to pass parameters to build (and remove debug line)
• [x] fixes for init and build when entry is draft (skip!)
• [x] update nbook and nimib.toml
• [x] update example
• [x] add tests
  • [x] test tocs (and entries)
  • [x] add to CI a test to create the example book from scratch

in a subsequent commit:

• [x] fix next/previous when there is a draft

[pietroppeter]

documenting and finalizing release will be done later