ropensci/tarchetypes: tar_rep_map() and logged skips

tarchetypes 0.2.1 Bug fixes

• Make the *_raw() target factories process command the same way whether it is an expression or ordinary language object.
• Ensure compatibility with targets 0.5.0.9000, which logs skipped targets.

• Add tar_rep_map() and tar_rep_map_raw() to perform batched computation downstream of tar_rep() (#50).
• Add tar_select_names() and tar_select_targets() to make certain metaprogramming tasks easier.
• In tar_map(), attempt to convert the elements of values into lists of language objects.