Parenthesize <Plug> mappings

Japanese Vim plugin authors wrap their <Plug> mappings like so:

        <Plug>(my_custom_mapping)

I believe this originated with Kana Natsuno with vim-textobj-user. This
style has the advantage of being visually unambiguous when used in
mappings.

I resisted doing this when I first started on this plugin because I
wanted to ensure that <Plug> map names like sexp_outer_list could be
looked up by a user using the K command.

However, I now realize that the K command happily does substring
searches, so adding ()s around the plug name does not affect the ability
to look for help docs.