Ignore trailing ! in proxy command name

Close #557

plug.vim

@@ -225,6 +225,7 @@ function! plug#end()
           endif
           call add(s:triggers[name].map, cmd)
         elseif cmd =~# '^[A-Z]'
+          let cmd = substitute(cmd, '!*$', '', '')
           if exists(':'.cmd) != 2
             call s:assoc(lod.cmd, cmd, name)
           endif

test/workflow.vader

@@ -471,7 +471,7 @@ Execute (PlugDiff):
 Execute (Trying to execute on-demand commands when plugin is not installed):
   call ReloadPlug()
   call plug#begin()
-  Plug 'junegunn/vim-easy-align', { 'on': ['EasyAlign', 'LiveEasyAlign'] }
+  Plug 'junegunn/vim-easy-align', { 'on': ['EasyAlign', 'LiveEasyAlign!'] }
   call plug#end()
 
   Assert exists(':EasyAlign')