Feature request : enable `cli` formatted message in assert_that

[yli110-stat697]

Hi,

We recently started to use cli package to format message/warning/error in our R package development work. To keep things consistent, we would want to format the message in assert_that(). Currently we have to replace all the assert_that() with if-else statement if we want to add formatting to the output message. It would be very nice if assertthat package can support styling message output too.