Add "research series" option in adj_inflation

[camille-s]

blscrapeR package uses BLS series CUSR0000SA0. To compare monetary values from before 2000, Census says to use CPI-U-RS, the research series. For example, base CPI had Connecticut's 1990 median household income as $78,276; using CPI-U-RS has $76,106. adj_inflation could take an argument to flag which of these to use, although I can't find a series number for the RS version.