vim: adopt a bunch of vimrc settings

From http://amix.dk/vim/vimrc.html
bash: add GOPATH
vim: change .po rewrap mapping
vim: change git-grep mapping
vim: change :nohlsearch mapping
Make downloads optional

Add command line param -f, which will call all the downloadable stuff.
Without it, the script will only make changes available locally.
Don't recreate symlinks if they already exist