This commit adds the tmux program to Home Manager.
In addition to configuring tmux, a user may specify tmux plugins from
Nixpkgs. These can be included in the list of `plugins` and can either
be a package (all tmux plugins live under `nixpkgs.tmuxPlugins.*`), or
an object which includes the plugin and an `extraConfig`, which will
be run immediately after sourcing the tmux plugin.
Finally, this commit introduces two nested programs which may be
enabled which depend on tmux: tmuxp and tmuxinator. These do not have
the ability to be configured, although this may be a future
contribution.
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