New vim plugins management

-   new submodule modules/vim-pathogen to handle plugins easily
    (see https://github.com/tpope/vim-pathogen)
-   new submodule modules/vim-nerdtree to see files tree in vim
    (see https://github.com/scrooloose/nerdtree)
-   new submodule modules/vim-mde-syntax for the MDE syntax
    (see https://github.com/markdown-extended/vim-mde-syntax)
-   new 'home/.vim/' directory to handle all this
-   the 'pathogen#infect()' method is now called in the .vimrc