* Requiring a file to be entered to finish editing
Running blackbox_edit_end without an argument doesn't currently print out a warning that no files are being changed. A developer in my team who was new to Blackbox committed a decrypted file (and made no changes to the GPG file) as they didn't realise the command hadn't worked.
The check I've added should help to avoid these errors.
* Adding argument check to start editing
This makes the beginning of all files the same and a little simpler.
`${0%/*}` turns "/home/user/repository/bin/blackbox_edit" into
"/home/user/repository/bin", exactly like basename but without eating a
process.
Because other scripts needed `$blackbox_home` I made this into a
standardard variable that's always available.
This also loads _stack_lib.sh always because _blackbox_common.sh
requires it.
