`destdir` argument for `create_package()` and `create_project()`

[rossellhayes]

create_from_github() includes a destdir argument specifying what directory to copy a project into. It also allows you to set a default destdir with options("usethis.destdir").

create_package() and create_project() currently don't have a destdir argument, meaning the user has to type out the full directory path each time they create a new project. If they had a destdir argument, they could use the same usethis.destdir option and users could ensure that their projects are always created in their chosen directory, whether cloned from GitHub or started from scratch.

If this would be helpful, I'd be happy to work on a PR.