Log Message:
(math/R-prettyunits) Updated 1.1.1 to 1.2.0

# prettyunits 1.2.0

* New `pretty_num()` function with all the [BIPM](https://www.bipm.org)
  agreed unit prefix (#26, @cregouby).

* New `pretty_round()` and `pretty_signif()` functions preserve the requested
  number of digits as character strings (#14, @billdenney).

* New `pretty_p_value()` function to convert p-values to character strings
  where small values are shown like "<0.0001" (#13, @billdenney).

* New `pretty_color()` functionm converts a color to a named color
  (#12, @billdenney).