Add capability to import all functions that start with a prefix

[hadley]

e.g. @importFrom purrr starts_with("map")

Probably would want to implement something close to tidyselection here.

I think we can built into @importFrom since there shouldn't be any instance where the second argument is a call.

[hadley]

Or maybe a regular expression would be sufficient.

[hadley]

No longer think this is important.