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/undotree.load.vim

@@ -1,4 +1,6 @@
 call dein#add('mbbill/undotree', #{
 	\	lazy: 1 ,
-	\	on_map: #{ n: [ '<Leader>u' ] },
+	\	on_cmd: [ 'UndotreeShow', 'UndotreeToggle' ],
 	\ })
+
+nnoremap <Leader>u :UndotreeToggle<CR>