rstantools v2.0.0

• Added Martin Lysy as a coauthor.

• New function rstan_create_package() (based on
  usethis::create_package()) replaces rstan_package_skeleton()
  for the purpose of starting a new package with Stan functionality.

• Stan functionality can be added to an existing package by calling
  use_rstan() instead of starting a new package from scratch.

• Stan folder infrastructure now puts all .stan files in inst/stan and all
  auto-generated C++ files directly in src. This last step ensures that custom
  Rcpp source code can coexist with the Stan C++ code.

• Each time a .stan file gets added/removed/modified requires a call to
  rstan_config() in order to generate the Stan C++ code and Rcpp::loadModule()
  calls. However, setting auto_config = TRUE (the default) in
  rstan_create_package() ensures rstan_config() is called whenever the package
  is installed (including via devtools::load_all()), so no need to call it
  manually unless the user wishes to inspect the Stan C++ code for issues.

• roxygen2 documentation is now optional, but remains the default.

• Rather than generating Stan "system files" via cat commands, rstantools
  now stores these as template files in inst/include/sys, so the build process
  can be easily modified as improvements become apparent.