ordered = TRUE for colorFactor fixes #800

[bogaotory]

When the code is written as pal <- colorFactor(c("navy", "red"), domain = c("ship", "pirate")), then it is highly likely that the user's intention is to have navy for ship, and red for pirate.
Therefore it makes more sense that colorFactor should assume that the domain vector is already ordered (ordered = TRUE) unless stated otherwise.

PR task list:

• Update NEWS (ordered = TRUE makes more sense for colorFactor #800)
• Add tests (where appropriate)
  • R code tests: tests/testthat/
  • Visual tests: R/zzz_viztest.R
• Update documentation with devtools::document()

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}