Support devtools help

This is a patch to display help pages of packages attached with `devtools::load_all`. Note that `devtools` overloads `utils::help`, therefore I explicitly call `help` from the `utils` namespace. Also note that devtools does not support to request a topic from a specific package, so I re-implemented the same heuristic as in `devtools::help` (check if the package is attached using devtools and then return first topic found in any of those packages).

I'm not sure what happens around line 39, maybe this is still not compatible with devtools. Yet basic "Rhelp [function]" seems to work for both types of package loading using the "stable" vim-r-plugin from vim-scripts.