Use rspec -P pattern option in rake tasks

rspec has an internal option (-P) to receive pattern of files to run via the command line, use this option in rake_tasks instead of passing through a huge list of files one by one on the command line.
What do you think about using `--pattern` rather than `-P`?  The short forms are nice when you are typing options at the command line but there's not that benefit here, and `--pattern` is more explicit about what it means (kinda how using single-letter variable names is generally frowned upon).