Drop support to GNU Screen, VimShell and Conque Shell.
The maintenance of so many different ways of running R and sending
commands to it was very complicated and was a source of bugs. People
wanting integration between the Vim-R-plugin and GNU Screen, VimShell
and Conque Shell will have to write extensions to the Vim-R-plugin
(vimrplugin_source). To simplify the code even further, the integration
with the screen plugin also was dropped.