Edit template

[malcolmbarrett]

This PR adds edit_template() for creating and editing files in inst/templates. I've resisted this PR because it's such a simple function, but I find myself constantly wanting to have it available. I'm conscious of the fact that, as the package matures, these little helper functions may not find their forever home in usethis. Nevertheless, here's why I think usethis should have this function:

1. use_template() introduces a structure, much in the way that R Markdown templates do. That structure should be automated away because if you misspecify it, templates will not work. use_rmarkdown_template() is important not just because of the complexity of that structure but the consistency. usethis templates use a considerably simpler structure, yet--and I'm not kidding--I can never remember if it's inst/template or inst/templates. I literally have to look it up every single time.
2. Templates are a key asset of usethis when you're using the package to develop internal tools. Templates are, in fact, probably the most useful feature of the ecosystem when you're building a usethis-like package. Iterating templates should thus have good support.

[jennybc]

I recently added a feature to pr_fetch() and pr_resume() that, when called argument-less, gives you a choice of PRs / branches.

I suspect something similar might be useful here, because who is going to remember the exact names of their existing templates? So if template is not provided, could say something like "looking up existing templates in inst/templates ... and provide a list.

[malcolmbarrett]

Ooo that's a great idea, thank you! I'll try to add that today. I've greatly enjoyed that functionality for the pr functions so far for exactly the same reason

[malcolmbarrett]

Ok, edit_template() now uses the same menu() approach as the PR functions, more or less. I'm excited about that!

PS I noticed that Hadley's change to the tests resulted in just calling edit_template() without capture.output(). I modeled that test after those for edit_file(); happy to do a small (separate) PR to make those tests consistent if capture.output() is not needed, if you like.

[jennybc]

Thanks!

[malcolmbarrett]

Awesome, thanks to you both!