[Gemspec] Add activesupport as development dependency

Just for #strip_heredoc
[SpecHelper] Make String#strip_heredoc ready
[Spec] Add unit test for redacting paths from Subject's execution output
[Refactor] Extract #write_output from Subject#launch
[Refactor] Extract #command_line from Subject#launch
[Refactor] Extract #run from Subject#launch
Fix Subject#replace_user_path if 2nd argument is left blank
[Refactor] Extract #replace_pattern from Subject#replace_path
[Refactor] Extract #apply_replacements from Subject#write_output
[Refactor] Extract #environment_var_assignments from Subject#run
Keep command line compatible