Interpret the second argument to Plug command as tag option

The older documentation of vim-plug suggested that the second string
parameter to Plug command was for both branches and tags. But we've
realized that tags and branches are not interchangeable (#174), and it
is now recommended that the user explicitly specify whether it's a
branch or a tag. The now-undocumented second parameter is currently for
branches, but I'd like to change it for tags, as the use of tags are
much more common.

plug.vim

@@ -439,12 +439,9 @@ function! s:parse_options(arg)
   let opts = copy(s:base_spec)
   let type = type(a:arg)
   if type == s:TYPE.string
-    let opts.branch = a:arg
+    let opts.tag = a:arg
   elseif type == s:TYPE.dict
     call extend(opts, a:arg)
-    if has_key(opts, 'tag')
-      let opts.tag = remove(opts, 'tag')
-    endif
     if has_key(opts, 'dir')
       let opts.dir = s:dirpath(expand(opts.dir))
     endif