Add codebook files to ipums_list_files() output

[robe2037]

Since codebook files can be read by ipumsr, it makes sense to include them in the list of available files in a zipped extract. This is primarily useful for NHGIS, which provides zipped extracts by default.

Further, with expanded NHGIS extent selection options, it's more likely to produce extracts that don't actually contain any data (selecting an extent that does not apply for a given dataset is currently allowable by the IPUMS API). In these cases, ipums_list_files() produces confusingly empty output if it does not include codebook files in the list. Importantly, the codebook files are particularly useful in this case, as they indicate if no data were available for the selected extents.

This PR adds codebook files (.txt for NHGIS, and—for completeness—.xml for microdata) to the output of ipums_list_files() and also removes arguments to this function that were deprecated in v0.6.0.