Handle keyboard shortcuts at plugin load time

Don't load plugins based on keyboard shortcuts, as this conflicts with
vim-which-key. Instead, create shortcuts when plugins are declared, and
load them when the commands they provide are used.

plugins/ctrlp.vim.load.vim

@@ -1,6 +1,8 @@
 call dein#add('ctrlpvim/ctrlp.vim', #{
 	\	lazy: 1,
-	\	on_map: #{
-	\		n: [ '<Leader>pi' , '<Leader>po', '<Leader>pp' ]
-	\	}
+	\	on_cmd: ['CtrlPBuffer', 'CtrlPMRUFiles', 'CtrlP']
 	\ })
+
+nnoremap <Leader>pp :CtrlPBuffer<CR>
+nnoremap <Leader>po :CtrlPMRUFiles<CR>
+nnoremap <Leader>pi :CtrlP<CR>