Add config for mackup

- ignore apps whose config files are in the `dotfiles` repo
  (see: https://gitlab.webartifex.biz/alexander/dotfiles)
- include various (config) files that should not be public:
  + Cisco's AnyConnect
  + history files for `bash`, `less`, `python`, `psql`, `tig`, and `zsh`
  + Nautilus' bookmarks
  + SSH config & known hosts

.mackup/README.md

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+# Custom Configuration for `mackup`
+
+This folder contains various **config** files
+    to include all kinds of files in the [`mackup backup`](https://github.com/lra/mackup).
+Their format is described [here](https://github.com/lra/mackup/tree/master/doc#add-support-for-an-application-or-almost-any-file-or-directory).
+
+`mackup` is used to synchronize (dot) files the general public should *not* see.

.mackup/anyconnect.cfg

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+[application]
+name = Cisco's AnyConnect
+
+[configuration_files]
+.anyconnect

.mackup/histories.cfg

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+[application]
+name = various history files
+
+[configuration_files]
+.bash_history
+.lesshst
+.python_history
+.tig_history
+.zsh_history
+
+[xdg_configuration_files]
+# `~/.config/psql` is set in `~/.psqlrc` for all history files
+psql

.mackup/nautilus.cfg

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+[application]
+name = GNOME Files a.k.a. Nautilus
+
+[xdg_configuration_files]
+gtk-3.0/bookmarks

.mackup/ssh_non-public.cfg

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+[application]
+name = non-public SSH (config) files
+
+[configuration_files]
+.ssh/config
+.ssh/known_hosts
+.ssh/known_hosts.old