Check if map exists before trying to unmap it.

If ds is not mapped even though vim-surround is loaded (perhaps one has
custom mappings), then when the rbrowser is started an annoying error
pops up.  Check if ds mapping exists before trying to unmap it.
Merge pull request #146 from mdlerch/surround_mapping

Check if map exists before trying to unmap it.
Allow the use of the old Python code to Send cmds
To use the old Python function SendToRConsole(), put in your vimrc:
let vimrplugin_libcall_send = 0
Make more mappings respect vimrplugin_user_maps_only

There were two that I noticed:

RSendPartOfLine:

Some infrastructure was already set up for this (RNRightPart,
RIRightPart, RNLeftPart, RILeftPart) were already mentioned in the doc
but not really set up.  I also changed the function a little.  Sending
right would send the right part including the cursor and sending left
would send the left part without the cursor.  Now they both send the
cursor.

RNextRChunk/RPreviousChunk:

These were stealing gn and gN which are actually built-in vim commands.
The RCreateMaps puts LocalLeader in front of the maps so these are now
\gn and \gN.  I considered adding another flag in a:type to not include
LocalLeader in the maps, but I decided to keep it simple.  Instead I
was going to make a note in the doc about the change of these maps
(addition of LocalLeader) and how to map without the LocalLeader (just
put the original code into ftplugin/rmd.vim), but I wasn't sure where a
good place for this would be in the doc.
Warn if +libcall feature is missing.
Merge pull request #147 from mdlerch/remove_unrequested_mappings

Make more mappings respect vimrplugin_user_maps_only
Merge remote-tracking branch 'upstream/master'
Change path of vimcom library.
Unset environment variable after R startup.
Finish the conversion of gn gN into \gn \gN.
Fix path of "cache" directory.
Get unbuffered string on R JobActivity
See: https://github.com/neovim/neovim/pull/1584
New highlight groups, as in colorout package
Split r-plugin/common_global.vim
Move part of the code of common_global.vim to other scripts.
Try to find SumatraPDF twice
ShowRDoc() now is called only by vimcom
Remove command :RpluginConfig
The vimcom package is setting the value of options("pager") and looking
for the presence of $VIMRPLUGIN_TMPDIR. I think that using Tmux all the
time is better than using the bash function "tvim". Consequently, the
configuration now is simpler.
Support multilib message and R History
Do the right thing again when the help for a topic is present in more
than one package. Show R history in Vim.
Call to SumatraFwd() on Windows
The code in the vimcom package is not working yet.
Use Sumatra command line instead of DDE.