Add `to_std(out|err)_from_any_process` matchers

These matchers will capture all output from spawned subprocesses in
addition to the main Ruby process.
Add docs and features around `to_std(out|err)_from_any_process` matchers
Add benchmarks around `to_std(out|err)_from_any_process` matchers
Change echo commands in output_spec if running on windows
Merge pull request #700 from alexgenco/output_matcher

Allow output matcher to capture output of a child process
Add changelog for #700.

[ci skip]