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/nerdtree.cfg.vim

@@ -1,6 +1,5 @@
 " NERDTree: ' e' = toggle side bar
 let g:NERDTreeGitStatusUpdateOnWrite = 1
-nnoremap <Leader>e :NERDTreeToggle<CR>
 
 " Set the bookmarks file's path
 let g:NERDTreeBookmarksFile = GetVardataPath( 'NTBookmarks' )