Ensure macOS compatibility for line- and block movement keymappings

The keymappings implemented in GH-31 works fine on Linux systems,
but conflicts with a global (non-modifyable) system keymapping on
macOS. Therefore a condition check for the currently running
environment should be used to use other specific keymappings for macOS
and Linux.

Resolves GH-172

snowblocks/vim/vimrc

@@ -160,13 +160,26 @@ map <C-n> :NERDTreeToggle<CR>
 " Moves the current line or selected block(s) up- or down by one line.
 "
 " In normal -and insert mode the "==" re-indents the moved line to suit its new position.
-" In visual mode "gv" reselects the last visual block while the "=" re-indents that block using the preassigned "'>" mark to identify the selection end.
-nnoremap <C-Down> :m+<CR>==
-nnoremap <C-Up> :m-2<CR>==
-inoremap <C-Down> <Esc>:m+<CR>==gi
-inoremap <C-Up> <Esc>:m-2<CR>==gi
-vnoremap <C-Down> :m '>+1<CR>gv=gv
-vnoremap <C-Up> :m '<-2<CR>gv=gv
+" In visual mode "gv" reselects the last visual block while the "=" re-indents that block using the
+" preassigned "'>" mark to identify the selection end.
+" Note that due to a global (non-modifyable) system keymapping on macOS it is necessary to use environment
+" specific assignments.
+
+if has("macunix")
+  nnoremap <C-S-Down> :m+<CR>==
+  nnoremap <C-S-Up> :m-2<CR>==
+  inoremap <C-S-Down> <Esc>:m+<CR>==gi
+  inoremap <C-S-Up> <Esc>:m-2<CR>==gi
+  vnoremap <C-S-Down> :m '>+1<CR>gv=gv
+  vnoremap <C-S-Up> :m '<-2<CR>gv=gv
+else
+  nnoremap <C-Down> :m+<CR>==
+  nnoremap <C-Up> :m-2<CR>==
+  inoremap <C-Down> <Esc>:m+<CR>==gi
+  inoremap <C-Up> <Esc>:m-2<CR>==gi
+  vnoremap <C-Down> :m '>+1<CR>gv=gv
+  vnoremap <C-Up> :m '<-2<CR>gv=gv
+endif
 
 "+---------------+
 "+ Configuration +