The command :PlugUpgrade will automatically clone the HEAD on master
from https://github.com/junegunn/vim-plug.git. As of today, the output
of `git describe master` is 0.9.1-45-g5fc9eab.
These features of vim-plug are particularly appealing:
* on-demand loading -> better startup times
* branch/tag/commit support -> controlled changes if desired
* support for unmanaged/local plugins -> good for development of plugins
