Add package parameter to use_template()

[hadley]

So it's actually useful in other packages.

[jennybc]

Is the goal also for a user to be able to mask built-in templates with their own? Example: user has their own preferred README template. If so, is that only via a custom package or ... should this somehow be configurable via a usethis.TEMPLATE option?

I suspect that adding the "package" parameter is good for now and the other stuff is just interesting to think about.

[hadley]

Right - this issue is just about making use template usable elsewhere.

[jennybc]

Closed in aab59f9