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.
