Add power_trace option to rspec command

[st0012]

Something like

$ rspec --power_trace

So the users won't need to toggle power_rspec_trace between tests.

(thanks @benoittgt for the idea!)

Implementation Plan

Patch https://github.com/rspec/rspec-core/blob/4fd54ca68ec66ebcaec2b7b5eb3d9c4fce5a5ae6/lib/rspec/core/option_parser.rb#L40

And to see if we can partially replicate the test here https://github.com/rspec/rspec-core/blob/d10e82d2aa83718a0fab2f3a421ff8a6b82762a6/spec/rspec/core/configuration_options_spec.rb