When should getOption("github.username") be checked?

[sboysel]

For example, the error for gists(what = "mineall") is somewhat cryptic until you realize the github.username is unset

> library(gistr)
> gist_auth()
<request>
Headers:
* Authorization: token abcdefghijklmnopqrstuvwxyz0123456789
> gists(what = "mineall")
Error: length(url) == 1 is not TRUE 

#66 encourages the user to manually set the option if gists(what = "mineall") is run when unset.

[sckott]

Thanks - looks like that's the only function that looks for the R option github.username so your PR sorts that out. After merging I'll add a note to the docs that users should make sure they have those options set.