Extend plug#load to process a list of names instead of varargs

Allows `call plug#load(keys(g:plugs))` for manually loading all plugins
at once. Close #638.

plug.vim

@@ -442,12 +442,13 @@ function! plug#load(...)
   if !exists('g:plugs')
     return s:err('plug#begin was not called')
   endif
-  let unknowns = filter(copy(a:000), '!has_key(g:plugs, v:val)')
+  let names = a:0 == 1 && type(a:1) == s:TYPE.list ? a:1 : a:000
+  let unknowns = filter(copy(names), '!has_key(g:plugs, v:val)')
   if !empty(unknowns)
     let s = len(unknowns) > 1 ? 's' : ''
     return s:err(printf('Unknown plugin%s: %s', s, join(unknowns, ', ')))
   end
-  let unloaded = filter(copy(a:000), '!get(s:loaded, v:val, 0)')
+  let unloaded = filter(copy(names), '!get(s:loaded, v:val, 0)')
   if !empty(unloaded)
     for name in unloaded
       call s:lod([name], ['ftdetect', 'after/ftdetect', 'plugin', 'after/plugin'])

test/workflow.vader

@@ -1141,6 +1141,12 @@ Execute (plug#load - invalid arguments):
   AssertEqual 0, plug#load('xxx', 'non-existent-plugin')
   AssertEqual 0, plug#load('non-existent-plugin', 'xxx')
 
+Execute (plug#load - list argument (#638)):
+  redir => out
+  call plug#load(keys(g:plugs))
+  redir END
+  AssertEqual '', out
+
 Execute (on: []):
   call plug#begin()
   Plug 'junegunn/rust.vim', { 'on': [] }