VIM: Changed plugin management to vim-plug

Before switching I used pathogen.vim for plugin management, which
resulted in a HUGE .gitmodules file as well as a huge directory
containing whole git repositories. vim-plug fixes all those problems. It
reduces the folder into a config file, containing one line per plugin
and also removes the need of adding those submodules to .gitmodules

Signed-off-by: Tobias Manske <tobias.manske@mailbox.org>

.gitignore

@@ -140,3 +140,5 @@ i3/config
 *.sqlite
 i3/scripts/dmenu/database.sqlite
 albert/*.db
+vim/plugged/
+vim/autoload/plug.vim