Do not trigger filetypeindent/filetypeplugin autocmds by default

[blueyed]

This is not necessary if filetype plugin indent on was not used before plug#end(), since then the FileType autocmds from there will come after vim-plug's.

This will issue a warning, and makes handling of this conditional.

This could use filetype plugin/indent off to work around this (similar to the filetype off being used), but runtime/indoff.vim and runtime/ftplugof.vim will only empty the augroups, and not remove them.

Fixing the user's config is the best solution anyway, so I think a warning is good.

Might help with https://github.com/junegunn/vim-plug/issues/581, too.

[blueyed]

Rebased and added queuing of autocommands before VimEnter in general (https://github.com/blueyed/vim-plug/commit/00d3f0ada65c0ce84263f9beb5d8daae3989aa7d, https://github.com/vim/vim/issues/2810).

[janlazo]

@blueyed Can you rebase again if it's useful to have?