Disable Fzf's native border

Fzf may be configured via `$FZF_DEFAULT_OPTS` to show a border, which
takes up unnecessary space in `fzf-lua`.

Setting `config.globals.fzf_opts['--border'] = 'none'` disables Fzf's
border.

README update: fzf default opt '--border=none'

README.md

@@ -401,6 +401,7 @@ require'fzf-lua'.setup {
     ['--info']        = 'inline',
     ['--height']      = '100%',
     ['--layout']      = 'reverse',
+    ['--border']      = 'none',
   },
   -- fzf '--color=' options (optional)
   --[[ fzf_colors = {

doc/fzf-lua.txt

@@ -446,6 +446,7 @@ Consult the list below for available settings:
         ['--info']        = 'inline',
         ['--height']      = '100%',
         ['--layout']      = 'reverse',
+        ['--border']      = 'none',
       },
       -- fzf '--color=' options (optional)
       --[[ fzf_colors = {

lua/fzf-lua/config.lua

@@ -135,6 +135,7 @@ M.globals = {
     ['--info']        = 'inline',
     ['--height']      = '100%',
     ['--layout']      = 'reverse',
+    ['--border']      = 'none',
   },
   previewers = {
     cat = {