New drake_script() function

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Proposal

i propose a new drake_script() function to write a _drake.R file. Example usage:

drake_script({
  source("R/packages.R")
  source("R/functions.R")
  source("R/plan.R")
  drake_config(plan)
})

This could help users transition to r_make(), as well as help implement #1281, ropensci-books/drake#160, and related issues.