Add option to enable colored output

[jimhester]

Right now if the autodetection fails you can set crayon options to turn on color for the rcmdcheck output, e.g. options(crayon.enabled = TRUE).

For the GitHub Actions use case it would be useful to have a rcmdcheck specific option or environment variable to control this, then the setup-r action can enable this without affecting color elsewhere in the workflow, e.g. hadley/mastering-shiny#132

[jimhester]

This would still be somewhat useful to have a rcmdcheck specific option to control the color, but for now we are setting the crayon option before running rcmdcheck(), which works ok.

[gaborcsardi]

Closed by #126, right @jimhester?

[jimhester]

yeah