useLatex as a book config

[pietroppeter]

• currently in order to add latex functionalities to a document you need to add nb.useLatex to each document
• we could add a useLatex configuration option in the book that activates this for the whole book

should be a rather easy implementation, in particular:

• add a new field (with documentation comment) in BookConfig type (in types.nim)
• a corresponding line should be added in useNimibook proc (in themes.nim)
• document change in book/content.nim (and should be auto documented in configuration)