add usethis:: like functions to allow data package construction interactively

[gfinak]

At the request of @raphg datapackage_skeleton() should be allowed to create
and empty skeleton directory.

• usethis::-like functions could be implemented to add data sets and processing scripts interactively.

Proposal:

• use_script

  • would take an R or Rmd file path.
  • copy the file into the skeleton
  • update the yaml
  • We could consider some way of tagging objects or code chunks or lines of code in the Rmd or R file
    to automatically extract names of data objects to add to the yaml config.

• use_data_object

  • would take a data object argument
  • would add the data_object to the yaml config.

• use_raw_dataset

  • would take a path to a directory or a specific file
  • would copy the directory or file into inst/extdata

Tagging data objects in code

• custom Rd like tag as comment followed by the name of the object:

#' @dataobject name

• need to write a parser?

Need to assess how this will interact with roxygen.. maybe not at all since we are parsing roxygen from documentation.R.