Allow NA estimates and MOEs

[civilstat]

In some of the 2018 ACS Ranking Tables (a prototypical use case for this R package), one or more US states have NA values for the estimate and/or MOE. We don't currently do any error handling for this.

Presumably if a user ran our functions on such a dataset, the user wouldn't want us to drop the incomplete geographies. Instead, we'd still like the tables and plots to list every state, but to write "NA" or show no points when the Est or MOE are missing.