Use vim-plug to load onedark instead of the two files in
vim/{autoload,colors}. This enables to update the theme and have it as a
submodule in the rep.
Replace the zsh-history-substring-search plugin with fzf by adding a
function that starts the fzf-history-widget when pressing <Up> with a
nonempty BUFFER. The widget then uses the buffer automatically as query
for fzf.
