Allow nonzero exit status via allowstatus option

In certain cases (notably pidof and ps), the nonzero exit
status cannot be avoided, while the successful execution is still desirable.

This patch adds :allowstatus option, in which the user can explicitly
enumerate allowed exit statuses. ExecutionFailed exception won't be
raised if the command returns one of these statuses.
Add documentation for allowstatus option

Update README and inline documentation.