Nixify fish configuration

This isn't 100% usable, but it works. You can build it with `nix-env` and then
run it with `wpcarros-fish`.

fish/default.nix

@@ -0,0 +1,52 @@
+{ pkgs ? import <nixpkgs> {} , ... }:
+
+# TODO: Is it appropriate to put programming language dependencies here? Should
+# I have a bin dependency for every fish `abbr` and `alias` that I use? What
+# makes the most sense?
+
+# TODO: Some of the abbreviations / aliases depend on binaries and some depend
+# on files (e.g. ~/.config/nvim/init.vim). How should I handle the file
+# dependencies?
+
+# TODO: Support symlinking config.fish to ~/.config/fish/config.fish using Nix.
+
+let
+  fishBinPath = pkgs.lib.strings.makeBinPath (with pkgs; [
+    # TODO: Support fasd instead of autojump.
+    # fasd
+    direnv
+    autojump
+    fzf
+    fd
+    xclip
+    bat
+    neovim
+    ripgrep
+    sdcv
+    exa
+    pass
+    networkmanager
+    google-chrome
+    docker
+    elixir
+    clojure
+    gnupg
+    git
+    tmux
+    # This is not that same as `hub`.
+    # git-hub
+    mercurial
+    stack
+    kubernetes
+    circleci-cli
+    nix # Really?
+    apt # Really?
+    pacman # Really?
+  ]);
+# TODO: It's difficult to test if the `--init-command` is working since fish
+# persists functions, abbreviations, aliases between sessions so it's easy to
+# get tricked by false-positives.
+in pkgs.writeShellScriptBin "wpcarros-fish" ''
+  export PATH="${fishBinPath}:$PATH"
+  exec ${pkgs.fish}/bin/fish --init-command 'source ${ ./functions.fish }'
+''