Implement use_standalone()

[hadley]

Fixes #1654

[hadley]

Requires r-lib/rlang#1548

[jennybc]

I realize this is a draft and is unfinished, but I made one pass on my way through the usethis notifications.

[jennybc]

Note to self: unskip "use_tidy_dependencies() isn't overly informative" test when this concludes.

[DavisVaughan]

I used it here and it worked well! tidyverse/dplyr#6722

[jennybc]

I think the main thing to settle is whether to make this work for GHE. I think we should do it, since we've met that standard everywhere else.

[jennybc]

With the caveat that I didn't manually test on GHE, yes, I think this is good to go (after you take my suggestion to pass host to standalone_choose()).

One last suggestion: in create_from_github(), perhaps we should clarify the docs a bit, now they are being inherited elsewhere.

Such as:

For create_from_github(), the REPO part will be the name of the new local folder, which is also a project and Git repo.

[hadley]

@jennybc the docs were a bit tangled, but I think I've made an improvement. Anyway, I'm going to merge this PR so it's off my plate, and you should feel free to tweak the docs as you see fit.