Add OSC 52 escape sequence yank

If the `tmux` `set-clipboard` option is set to `on` add the `osc52yank`
plugin and change `<leader>y` mapping to use it.

plugin/mappings.vim

@@ -76,11 +76,15 @@ nnoremap <leader>tm :tabmove<Space>
 " Clear search highlights
 nnoremap <leader><Space> :nohlsearch<CR>
 
-" System clipboard yank/put
-noremap <leader>y "+y
-noremap <leader>Y "+Y
-noremap <leader>p "+p
-noremap <leader>P "+P
+if g:use_osc52
+  noremap <leader>y :Oscyank<CR>
+else
+  " System clipboard yank/put
+  noremap <leader>y "+y
+  noremap <leader>Y "+Y
+  noremap <leader>p "+p
+  noremap <leader>P "+P
+endif
 
 " Quickly access spelling menu
 inoremap <C-s> <C-g>u<C-X>s