Opt-out bookdown environment definition for lipic article

[cderv]

This fixes #392 (cc @nuest)

This can now be used in YAML

output:
  bookdown::pdf_book:
    base_format: rticles::lipics_article

and it will no more error with

! LaTeX Error: Command \theorem already defined.
               Or name \end... illegal, see p.192 of the manual.

This change set an option for opting out the addition by bookdown of some definition. This requires bookdown 0.23 (currently dev version) to work

remotes::install_github("rstudio/bookdown")

@yihui

• is this how you would do it ? I used the on_exit argument to be sure to reset the options after render()
• I did not set a requirement for bookdown up in DESCRIPTION as it is in suggest, should I ?
• This requires bookdown 0.23, should we merge in rticles only when it is release ? I added a mention in NEWS about the requirement for this.

[nuest]

:tada: