After I installed vim-airline, I found that there are a variety of plugins in the path
vim-airline\autoload\airline\extensions
but only a part of them are loaded every time I start Vim.
How can I enable extensions such as syntastic, tagbar, unite, ctrlp and so on?
A possible way is to copy the .vim file to 'plugins' folder that vim provides. However, I want a solution that airline provides so that I don't have to manipulate files and can just configure them by a few commands.
I have been confused with how those extensions work for a few days.
It turns out that I have misunderstood what those extensions are.
Extensions come with airline are not plugins themselves, but some additional features related to those plugins to be displayed in the statusline.
That is to say, one has to install those plugins first before he enables those extensions. And after the installation of plugins, airline can work perfectly with them, providing cool style to display their status.