[UPDATE][VIM] Move mappings to different files

Because a lot of my mappings use the <Leader> key, this is the only file
that *must* be loaded before all the others, otherwise the value of
leader when the script is read will be erroneous.

I also tried to use `git write-tree` and `git commit-tree` to emulate a
manual octopus merge to preserve the line history for each file. But
honestly that gave a graph that was too ugly for a history that really
did not need that amount of accountability. However it is a fun
exercise, and I recommend to everyone to try it at least once.
See Raymond Chen's blog `The Old New Thing`, with the post titled `How
to split out pieces of a file while preserving git line history: The
hard way with commit-tree`, for an explanation.

vim/.vim/plugin/mappings/leader.vim

@@ -0,0 +1,6 @@
+" Map leader to space (needs the noremap trick to avoid moving the cursor)
+nnoremap <Space> <Nop>
+let mapleader=" "
+
+" Map localleader to '!' (if I want to filter text, I use visual mode)
+let maplocalleader="!"