Append original CRAN metadata to recipe.

[jdblischak]

To address #25, this PR extracts the original CRAN metadata and inserts it after the maintainer list.

cc: @mingwandroid @bgruening

As an example, the bottom of the meta.yaml for r-usethis would be:

extra:
  recipe-maintainers:
    - johanneskoester
    - bgruening
    - daler
    - jdblischak
    - cbrueffer
    - dbast

# Package: usethis
# Title: Automate Package and Project Setup
# Version: 1.4.0
# Authors@R: c( person("Hadley", "Wickham", , "hadley@rstudio.com", role = c("aut", "cre"),  comment = c(ORCID = "0000-0003-4757-117X") ), person("Jennifer", "Bryan", , "jenny@rstudio.com", role = "aut", comment = c(ORCID = "0000-0002-6983-2759") ), person("RStudio", role = c("cph", "fnd")) )
# Description: Automate package and project setup tasks that are otherwise performed manually. This includes setting up unit testing, test  coverage, continuous integration, Git, 'GitHub', licenses, 'Rcpp', 'RStudio'  projects, and more.
# License: GPL-3
# URL: https://github.com/r-lib/usethis
# BugReports: https://github.com/r-lib/usethis/issues
# Depends: R (>= 3.1)
# Imports: clipr (>= 0.3.0), clisymbols, crayon, curl (>= 2.7), desc, fs (>= 1.2.0), gh, git2r (>= 0.23), glue, rlang, rprojroot (>= 1.2), rstudioapi, utils, whisker
# Suggests: covr, knitr, magick, rmarkdown, roxygen2, spelling (>= 1.2), styler (>= 1.0.2), testthat (>= 2.0.0), withr
# Encoding: UTF-8
# Language: en-US
# LazyData: true
# RoxygenNote: 6.1.0
# NeedsCompilation: no
# Packaged: 2018-08-14 05:17:52 UTC; jenny
# Author: Hadley Wickham [aut, cre] (<https://orcid.org/0000-0003-4757-117X>), Jennifer Bryan [aut] (<https://orcid.org/0000-0002-6983-2759>), RStudio [cph, fnd]
# Maintainer: Hadley Wickham <hadley@rstudio.com>
# Repository: CRAN
# Date/Publication: 2018-08-14 12:10:02 UTC

[bgruening]

Thanks @jdblischak!