Allow tidyselect-like syntax within `use()`

This could be seen as an extension of #287.

E.g. `use(purrr[contains("map")])` would import `map()`, `imap()`, `map_chr()`, `map_dbl()` etc. Another common pattern might be `use(ggplot2[ggplot, aes, starts_with("geom_")])`. In my opinion this would do a lot for readability/convenience, and would lend familiarity for those who already use the tidyverse.

The only possible issue I can see is that tidyselect would use `everything()` rather than `...` to import the whole namespace. I suppose supporting both might be possible, but perhaps not very neat.

Start by reading issue #287 and the current use() behavior. Work out the intended semantics for tidyselect-like brackets, including how contains(), starts_with(), ..., and everything() should behave. Done means the purrr and ggplot2 examples select the expected exports with documented, tested behavior.

Written by the indexing model from the issue text.