Adding a function unit_format

This function allows users to add units to labels. 

unit_format(unit = "m")(c(1e3, 2e3)) gives c("1,000 m", "2,000 m")
unit_format(unit = "m", sep = "")(c(1e3, 2e3)) gives c("1,000m", "2,000m")
unit_format(unit = "m", scale = 1e-3)(c(1e3, 2e3)) gives c("1,000m", "2,000m")