Replace :focus_on_failed and :keep_failed options with :failed_mode one

New :failed_mode values explained:

* :focus (default) *
Focus on the first 10 failed specs, rerun till they pass.
It reproduces old :focus_on_failed behaviour.

* :keep *
Keep failed specs until they pass (add them to new ones).
It reproduces old :keep_failed behaviour.

* :none *
Just report.
Move lib/guard/rspec/formatters/formatter.rb to lib/guard/rspec/formatter.rb
Fix KeepingInspector: Do not #_clean @failed_locations