Add desc argument to fct_infreq()

[chrislongros]

Closes #365.

Adds a desc argument so you can order levels by ascending frequency with desc = FALSE. Defaults to TRUE so nothing changes for existing code.

f <- factor(c("b", "b", "a", "c", "c", "c"))
fct_infreq(f)              # c, b, a (descending, same as before)
fct_infreq(f, desc = FALSE) # a, b, c (ascending)

Includes docs, an example, and tests. R CMD check passes with 0 errors, 0 warnings, 0 notes.