Set option for destdir

[malcolmbarrett]

In create_from_github(), use_course(), and use_zip(), usethis tries to guess a good download location if destdir is NULL. I think it would be nice to also set an option in your .Rprofile to specify a default location, e.g. usethis.destdir. I believe this would only entail a simple change to conspicuous_place(), which would check if such an option is set and return that location if so, otherwise going on with its usual business. Happy to make a PR if agreeable!