add a cpi_deflate() function

[MattCowgill]

Uncertain about the name. Similar to grattan::cpi_inflator() by @HughParsonage but (1) won't use projections for future dates; and (2) will use the non-seasonally adjusted headline CPI.

[HughParsonage]

Don't make the same mistake I did with grattan::cpi_inflator's basically useless first argument.

[MattCowgill]

Cheers @HughParsonage. I'm not entirely sure I'll do this. Have you considered breaking out your *_inflator() functions into their own package (like you did with {fy})?