maint: add a 'print' matcher to rspec, to inspect std{out,err}

You can now write expectations along the lines of:

expect { ... }.to print /whatever/

This will do the expected thing, which is to require that we print something
matching that regular expression during the block. The output itself is
suppressed during operation of the matcher.

Paired-With: Max Martin <max@puppetlabs.com>