Add g:fzf_vim.options for fzf options shared by all commands (#1624)

- Key hint footer had no off switch. $FZF_DEFAULT_OPTS could not give one,
  since fzf reads it before the options fzf.vim passes
- Merged ahead of the per-call and command-level options, so both still
  take precedence
- '--no-footer' there hides the hints. Documented as fzf-vim-key-hints,
  the footer had no name to look up
- Help TOC also gained Paste key and Show key, missing since those
  sections were added

Close #1623

autoload/fzf/vim.vim

@@ -400,6 +400,9 @@ function! s:fzf(name, opts, extra)
   let extra  = copy(extra)
   let eopts  = has_key(extra, 'options') ? remove(extra, 'options') : ''
   let merged = extend(copy(a:opts), extra)
+  " Global fzf options
+  call s:merge_opts(merged, s:conf('options', []))
+
   call s:merge_opts(merged, eopts)
 
   " Command-level fzf options