Add 'g:nerdtree_execute_cmd' command setting

[syurazo]

When user wants to use a command like "xdg-open", they can add g:nerdtree_execute_cmd variables and modify the command.

if s:is_plugged("nerdtree-execute")
  if executable('xdg-open')
    let g:nerdtree_execute_cmd = 'xdg-open'
  endif
endif