Treat unset variables and parameters as an error.

manpage:

> Treat unset variables and parameters other than the special parameters
> "@" and "*" as an error when performing parameter expansion.  If
> expansion is attempted on an unset variable  or  parameter, the shell
> prints an error message, and, if not interactive, exits with a non-zero status.
Do not ignore failing commands but exit.
Merge pull request #3 from syranez/master

Sane bash script development settings