POC: Adding bash-completion to byobu.

* Linking repositories containing the files for future traceability (License: GPLv2)
* Symlinking the required files
* Adding bash-completion to byobu executables:
  * Hard-coding `byobu-tmux` and `byobu-screen` to respected complete-functions
  * Small shim to redirect on the correct bash-completion function
    in case of using the plain `byobu` (as expected on the normal usecase)

Tested under Ubuntu 16.04.3.

_Note that unfortunately, I didn't have screen installed to test or verify the actual
usage of `screen` file. I am not familiar with `screen` commands, but I was seeing
bash functionality "completed". However, autocomplete changed realtime to the
contents of `source`d file._

Signed-off-by: Ntentos Stavros <133706+stdedos@users.noreply.github.com>

.gitmodules

@@ -0,0 +1,6 @@
+[submodule "bash-completion"]
+	path = bash-completion
+	url = https://github.com/GArik/bash-completion.git
+[submodule "tmux-bash-completion"]
+	path = tmux-bash-completion
+	url = https://github.com/imomaliev/tmux-bash-completion.git

bash-completion

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+Subproject commit afe39fd1e171d0ea3d4fb8ec0d8c8c14fa120ed8

etc/bash_completion.d/byobu

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+#!/bin/bash
+
+_byobu() {
+	source <(sed 's/^/local /' "${HOME}/.byobu/backend")
+
+	if [ "${BYOBU_BACKEND}" == tmux ] ; then
+		_tmux "$@"
+	elif [ "${BYOBU_BACKEND}" == screen ] ; then
+		_screen "$@"
+	else
+		return 1
+	fi
+}
+
+complete -F _byobu byobu
+complete -F _tmux byobu-tmux
+complete -F _screen byobu-screen

etc/bash_completion.d/screen

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+bash-completion/completions/screen

etc/bash_completion.d/tmux

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+tmux-bash-completion/completions/tmux

tmux-bash-completion

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+Subproject commit 847eda60c4664bfca833a086a3e2167957c26be0