Support crayon codes in `complete` and `incomplete`

[josswright]

I found in issue #32, and the related pull request #52, that progress supports colour codes from crayon in its format strings.

I was trying, though, to use colour codes for the progress bar itself via the incomplete and complete options. This gives me an error due to the resulting string being not equal to 1: Error in assert_single_char(incomplete) : nchar(x) == 1 is not TRUE.

Is this easily fixable? It would make my progress bars much prettier!

[HenrikBengtsson]

I've created PR #118 for this. It'll allows you to do:

library(progress)
library(crayon)
pb <- progress_bar$new(width=40L, complete=bgBlue(" "), incomplete=bgCyan(" "), current=bgCyan("|"))

resulting in: