Why not return the number of failed tests from Invoke-Pester

I tend to call Invoke-Pester from within build scripts, so being able to
just simply get the number of failed tests would be great!

Please review this however, I wasn't sure if it would be possible for
the $pester or the $pester.TestResults objects to be null at this point
in the execution, because there are no tests for the cmdlets within the
.psm1