Add a simple bash completion helper

tools/bash_completion/README

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+Bash Programmable Completion for Claws Mail
+-------------------------------------------
+
+This directory contains a script which provides argument completion
+when using Claws Mail from the command line under the bash shell.
+This is usually triggered by pressing the tab key (↹).
+
+See bash manual (https://www.gnu.org/software/bash/manual/) for more
+details about programmable completion.
+
+Usage
+-----
+
+Examples below assume current directory is the toplevel directory of
+either an extracted tarball or a git clone.
+
+Single user
+----------
+
+For a single user just make a directory (e.g. '.bash_completion.d')
+on your $HOME directory and copy the completion script there:
+
+$ mkdir ~/.bash_completion.d
+$ cp tools/bash_completion/claws-mail ~/.bash_completion.d/
+
+Then, source the script to enable it:
+
+$ . ~/.bash_completion.d/claws-mail
+
+To enable it permanently just add the command to the user's bashrc:
+
+$ echo '. ~/.bash_completion.d/claws-mail' >> $HOME/.bashrc
+
+All users
+---------
+
+For system wide installation just copy the file to the system directory,
+which is usually located in /etc (you need to become root for that):
+
+$ sudo cp tools/bash_completion/claws-mail /etc/bash_completion.d/
+
+After login in again all users should be able to use completion.

tools/bash_completion/claws-mail

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+# claws-mail(1) completion
+_claws-mail()
+{
+	local cur prev words cword
+	_init_completion || return
+
+	case $prev in
+	--help|-h|--version|-v|--version-full|-V)
+		return
+		;;
+	--alternate-config-dir)
+		COMPREPLY=( $( find . -maxdepth 2 -name clawsrc | sed 's,/clawsrc,,' ) )
+		return
+		;;
+	--select|--status|--status-full)
+		_filedir -d
+		return
+		;;
+	--compose-from-file|--attach)
+		_filedir
+		return
+		;;
+	esac
+
+	if [[ $cur == -* ]]; then
+		COMPREPLY=( $( compgen -W '$( _parse_help "$1" )' -- "$cur" ) )
+		return
+	fi
+} &&
+complete -F _claws-mail claws-mail