Allow customization of /bin/bash shell options

[fago]

It would be really nice if phapp.yml could be configured to run /bin/bash with "/bin/bash -e" so to have the effect of "set -e

It would also be nice to be able to use "-x" as well.

[fago]

The system environment (phapp config file) should define where bash executable is found.
But the project wants to define the default shell options (arguments to bin/bash or set call)