Optionally load named lists of sub-targets

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.

Proposal

Automatic concatenation with vctrs::vec_c() can make dynamic branching confusing in situations like ropensci-books/drake#142. loadd(subtarget_list = TRUE) and readd(subtarget_list = TRUE) should load/return lists of sub-targets instead.