Introduce config.set_proc_title

`config.set_proc_title` is a setting to enable a feature of example to set the
title of process of rspec to be the current example location.

If config.set_proc_title set to true, the process title is set to the current
example location and the full description of the example.

If config.set_proc_title set to `:simple`, the process title is set to the
current example location.