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/dein-ui.vim.load.vim

@@ -1,6 +1,8 @@
 call dein#add('wsdjeg/dein-ui.vim', #{
 	\	lazy: 1,
-	\	on_map: #{
-	\		n: [ '<Leader>Pu' , '<Leader>Pi' ]
-	\	}
+	\	on_cmd: ['DeinUpdate'],
 	\ })
+
+" Add key mappings for updates and installation
+nnoremap <silent> <Leader>Pu :DeinUpdate<cr>
+nnoremap <silent> <Leader>Pi :call CheckDeinInstall()<cr>