Use option or environment variable for imgur_upload.

[jonthegeek]

Closes #2233.

I implemented it like xfun::tinify(), but I'm not sure if it makes sense to involve opts_knit here.

[cderv]

Not sure about the CI error, seems like something is happening with pak. Maybe transient...

Regarding opts_knit, this would still be valid and it is the current way of providing a key. reprex could have leveraged that for example. See in https://github.com/tidyverse/reprex/blob/77ec1f612ea3d94534d6b37336a45f2d2957f3b2/R/reprex_document.R#L73-L77

reprex define the use of imgur and could have offer a way to configure this using reprex options and the key argument.

However, I find it a good idea to get option or/and env var from the knitr function directly.

I let @yihui do the review for his thoughts.

[yihui]

Thanks!