.vimrc: disable delimitMate plugin for text files
.ctags: index filenames too

The 'f' flag means (from man ctags):
  Include an entry for the base file name of every source file (e.g.
  "example.c"), which addresses the first line of the file.
.ctags: add language definition for .nix files

This is included in the ctagsWrapped expression in nixpkgs, but it's
nice to have it available independent from nixpkgs.
.ssh/config: update