build helptags on opening vim

This possibly is not a good idea, but I don't notice any slowdown yet.
add search-and-replace git alias

This and other idiosyncratic aliases don't belong in the base class.
add named sections to git::ignore

And fix it for single ignore values.
git::ctags
gitignore vim swapfiles
split tmux.conf into topical classes
use insecure and secure ssh keys