source_gist: gist.github.com now has username in URL

[wch]

The old url format was https://gist.github.com/1654919 but it has now changed to https://gist.github.com/kohske/1654919.

[wch]

Also, it should allow private gists, which can have hexadecimal characters. See the Shiny implementation of runGist for code that deals with both these issues:
https://github.com/rstudio/shiny/blob/master/R/run-url.R#L25